OSLO (Reuters) - A number of groups have expressed interest in Norwegian Air after British Airways-owner IAG said it may want to buy the budget carrier, Norwegian chief executive and founder Bjoern Kjos told Reuters on Thursday.

IAG announced on April 12 it had acquired a 4.6 percent stake in the airline as a platform for starting talks for a full offer, in a deal that could expand IAG's budget offerings and give it control of a struggling rival.

"What I can say is that these are very serious players and that the board is handling it. When I say serious players, the most obvious thing to think of is airlines," Kjos said in an interview on the sidelines of an earnings presentation.

"But there could also be others, as we've had multiple players showing interest. They are very serious players."

Shares in Norwegian Air were up 9.5 percent at 0830 GMT. It was the best performing stock on the Oslo bourse benchmark index <.OSEBX>, which was up 1.3 percent.

Since the start of the year shares in the airline are up 52 percent, mostly thanks to IAG's interest in the company. It now has a market capitalisation of around 13 billion Norwegian crowns (1.15 billion pounds).

Kjos told reporters he would prefer not to sell his stake until the company's intercontinental business had demonstrated its potential, while adding that he was unlikely to block a sale if the board were to "strongly recommend" a bid.

Kjos and Norwegian Air Chairman Bjoern Kise are the top shareholders in Norwegian Air via the HBK Invest Holding firm which owns around a quarter of the company.

"Although these inquiries may not be as serious as IAG's, interest from other parties could drive IAG's acquisition price higher," said Davy Research.

Germany's Lufthansa declined to comment when asked whether it was one of the parties interested in Norwegian.

Norwegian Air is in the midst of a massive expansion of its transatlantic operations as it tries to replicate the budget model that worked for European flights.

The strategy has led to higher costs, deepening losses, and a share issue in March to try to shore up the company's balance sheet.

Norwegian Air said its losses before interest and taxes deepened by 31 percent to 2.2 billion crowns year-on-year, while its revenues grew by a third over the same period to 7 billion crowns.

Norwegian Air had warned of a bigger loss to cope with higher fuel costs on March 20.

It also reiterated its full-year outlook.

(Editing by Terje Solsvik and Keith Weir)

By Joachim Dagenborg and Gwladys Fouche