AMSTERDAM (Reuters) - Dutch telecom group KPN NV (>> KPN KON) hit back at a surprise stakebuilding attempt by Mexico's America Movil (>> America Movil SAB de CV), saying it was substantially undervalued by the 3.2 billion euros ($4.2 billion) plan designed to give the Mexican group a base for possible European expansion.

America Movil, controlled by tycoon Carlos Slim, is seeking a stake of up to 28 percent in KPN and sees it as a long-term investment that would give it a presence in Europe at a time when the Mexican group has run out of opportunities to expand at home.

But KPN said the offer was pitched too low. "KPN is of the opinion that 8 euros per ordinary KPN share substantially undervalues the company," it said in a statement on Tuesday. "KPN will seek further clarification as to America Movil's intentions ... In the meantime, KPN will explore all strategic options."

If successful, the deal could give America Movil seats on KPN's supervisory board and would grant it a presence in Germany, where KPN has been trying unsuccessfully for years to merge its E-Plus unit with Telefonica-owned (>> Telefonica SA) O2 Germany.

KPN shares surged more than 20 percent to their highest since early April, bouncing from a seven-year low set earlier this month.

"It's management's first instinct to say an offer doesn't reflect the full value of the company. But ... valuation ... is not determined by management, but by investors and analysts," said Ulrich Rathe at brokerage Jefferies in London.

Rathe said a hostile offer would not make sense if America Movil wanted to work with KPN management. And asked if he thought KPN could try to enlist a white knight, he said there were few likely candidates. "Clearly this offer came from left field. I can't envisage another candidate who would take on Carlos Slim. There isn't a long list."

Yet the entry of Slim as a major investor will not solve the structural problems KPN has been grappling with, including tough competition in its home market and a lack of critical scale in its remaining foreign markets.

America Movil, which has already built up a 4.8 percent stake in KPN by buying shares in the open market, is the biggest mobile operator in Latin America and a major cash cow for Slim.

The Mexican tycoon, ranked by Forbes magazine as the world's richest man, has built his empire by purchasing troubled companies and turning them around.

"America Movil is a long-term investor, we think if the company (KPN) executes (its strategy) well, it will perform well," Carlos Garcia Moreno, America Movil CFO, told reporters, adding there was scope to cooperate in areas like roaming, content, marketing and procurement.

Garcia said it was too early to say whether America Movil would do other deals in Europe, but suggested KPN would allow it to get a closer look at European markets.

"KPN is the target for our first investment ... We have a long-term investment horizon. We've taken our time. This one seems to make a lot of sense," he told reporters.

BID PRELUDE

Some analysts said the move could be a prelude to a full bid. "We believe the 28 percent stake could be a first step to try to gain full control of KPN," SNS Securities said in a note. "Buying an incumbent operator is politically not without risk, which may explain the cautious approach of first acquiring a 28 percent stake."

One investment banker involved in the telecom sector said Slim had been looking to invest in Europe for several years.

"What they achieved today really matches their strategy, as they always wanted to start with a moderate investment and see what happens," the banker said.

"It would make no sense for Slim to make a full bid for KPN right now. Instead, it is very shrewd from a non-European company to start with a minority stake and increment it over time ... From Slim's perspective, the stock is cheap and it's a good asset."

One European telecoms executive said the move was an inexpensive way for America Movil to enter Germany, where E-Plus is the second-smallest player. "You get Germany at a very, very low price," he said.

He added the entry of Slim as a major shareholder would almost certainly end any remaining ambitions for a merger of E-Plus and O2, whose parent Telefonica is a major rival to America Movil in Latin America. "If I was Carlos Slim I wouldn't buy 28 percent of KPN in order to get rid of E-Plus."

Shares of KPN, which traces its origins back to the Dutch government's construction of telegraph lines in 1852, hit a seven-year low earlier this month. They traded up 17.1 percent at 7.592 euros by 1533 GMT. America Movil was down 6 percent at 17.54 pesos, after touching a six-month high on Monday.

KPN, which has 45 percent market shares in the Netherlands in fixed line and mobile, posted core earnings of 5.1 billion euros in 2011 and free cash flow of 2.45 billion.

But the company has been hit by a string of problems under Chief Executive Eelco Blok, a keen sailor and KPN-lifer who took the helm in April 2011, and has faced criticism from analysts, regulators, politicians and the public.

Ulrich at Jefferies noted KPN had underperformed the sector on a total return basis by 29 percent over the past year. "That is relative to a sector which has suffered a lot," he said.

INTENSE COMPETITION

KPN has been struggling to reverse a decline in revenue, profit and market share in its fixed-line and mobile operations as it faces intense competition on its home turf. Its chief financial officer unexpectedly quit in January, citing disagreements over internal governance.

KPN and other mobile phone operators in the Netherlands are under antitrust investigation for possible price-fixing, while the local telecoms regulator put KPN under close supervision in December saying it may have broken the law to the detriment of consumers and competitors.

Another potential negative is that the Netherlands may get a fourth mobile operator at the next auction of mobile licenses later this year, since the regulator has set aside a chunk of spectrum at a low price for a new entrant.

Further competition could mean KPN's mobile business in its key home market will become structurally less profitable.

Under pressure from shareholders to improve performance, KPN has started to look at divestments including the possible sale of its Belgian subsidiary.

Sources familiar with the company's plans say KPN is mulling the sale of BASE, Belgium's smallest operator, and wants 1.8 billion euros for it.

Earlier this month, Der Spiegel reported that E-Plus was in early talks to sell thousands of cell phone towers to a financial investor to raise funds for network expansion.

Those divestments could be put on hold if America Movil wants to use KPN as a foothold for expansion in Europe, one person familiar with KPN's thinking said.

KPN has been rumored as a takeover or merger target in the past, most recently in September 2011 when Belgacom (>> BELGACOM) said a merger with KPN could make sense.

The Dutch firm has snubbed three bid attempts, KPN's former chief executive Ad Scheepbouwer said last year, but he declined to give names of the companies involved.

MOST CITED

Spain's Telefonica has been most often cited as a possible buyer of KPN. The two firms held merger talks in 2000 which collapsed after KPN said it felt the Telefonica board was not committed to the proposed link-up.

A source familiar with KPN's M&A talks said it had continued to seek tie-ups or strategic investors to boost its performance, including Chinese telecoms companies.

KPN is struggling to hold on to its market share as it invests in infrastructure in the Netherlands. As part of a major cost-reduction plan it aims to shed between 4,000 and 5,000 full time jobs, or up to 16 percent of the total, by the end of 2013.

America Movil, which said it already owns 4.8 percent of KPN's stock, said it would make a cash offer of 8 euros per share for the additional stock, a premium of roughly 23 percent to Monday's closing KPN share price.

Maurice Mureau, asset manager at Dutch brokerage and asset management firm Keijser Capital, said he did not expect a rival offer to appear.

"At this price, this is a good moment to say goodbye to KPN shares, so we are selling half our stake this morning," Mureau said. "KPN's business model is under pressure. They are losing share in the traditional telephone market and the new business in internet is not fully compensating for that.

"The competition is only getting tougher and things could go any direction," Mureau added. "All-in-all a good time to get out of the stock."

Deutsche Bank is advising America Movil on the KPN deal, which would also involve handling the tender offer if that goes ahead, while Clifford Chance is advising it on legal matters.

The deal would be the second major move on a Dutch company in recent months by an acquirer based in the Americas, after United Parcel Service Inc (>> United Parcel Service, Inc.) announced plans to buy TNT Express NV (>> TNT EXPRESS) for 5.2 billion euros.

Previous Mexican investments in the Netherlands include a deal sealed in February for plastic pipe maker Mexichem (>> MEXICHEM SAB de CV) to buy Wavin (>> WAVIN) for 531 million euros.

($1 = 0.7663 euros)

(Aditional reporting by Anthony Deutsch; with Robert-Jan Bartunek in Brussels; Sophie Sassard and Georgina Prodhan in London; Ioan Grillo, Tomas Sarmiento and Dave Graham in Mexico City; Editing by Richard Pullin and David Holmes)

By Gilbert Kreijger and Sara Webb