Major banks ranging from ING (>> ING GROEP) to HSBC (>> HSBC Holdings plc) and Deutsche Bank (>> Deutsche Bank AG) put big bets on oil when record crude prices made even the most hazardous project look economically viable. But over the past year and a half, oil has slumped to near 12-year lows, spreading pain across financial markets.

Now with some energy projects facing the threat of being shut down, banks may see the pain turning into losses or eating into their capital strength.

The problem does not look confined to North America, where energy exposure is greater and big banks such as Citigroup (>> Citigroup Inc) and Bank of America (>> Bank of America Corp) have already disclosed billions in provisions.

"Investor concerns have turned to Europe," said Michele Pedroni, fund manager at SYZ Asset Management in Geneva. "The problem could be painful, but even if there is limited visibility for now it seems to be manageable."

Owners of European bank shares - already battered by other worries, such as negative interest rates - will be watching as the deadline for big loan repayments approach. Persistently low oil prices could also worsen oil firms' creditworthiness.

Strategists at BNP Paribas estimate European banks have a combined 400 billion euros ($445 billion) of loans to the energy sector. A fifth are rated as high yield, possibly resulting in 6 billion euros of losses through defaults.

A surge in the cost of credit protection, through credit default swaps, could also force banks to "mark to market" - write down the value of the debt - potentially leading to higher provisions for non-performing loans, the BNP Paribas team said.

European banking stocks <.SX7P> have fallen 20 percent this year, nearly twice as much the broader market, on fears about slowing economic growth and record-low interest rates, as well as concern over cheap oil.

"Valuations in many cases reflect risks from the energy and commodity complex," said Morgan Stanley. It urged caution on Standard Chartered (>> Standard Chartered PLC), DNB (>> DNB ASA), Credit Agricole (>> CREDIT AGRICOLE), Natixis (>> Natixis), ING and ABN (>> ABN AMRO Group), with oil's decline potentially cutting capital levels as much as 175 basis points.

The oil price slump has added to concern over low inflation, prompting the European Central Bank to cut rates, which in turn is putting more pressure on bank margins.

Fund managers such as Veronika Pechlaner at Ashburton and Enrico Vaccari at Consultinvest said focusing only on oil exposure offers only a partial view of the ugly context in which European banks are operating. But more disclosure over their energy loan book would be welcome, they said.

Lenders in Europe have been less systematic than their U.S. peers in announcing provisions, analyst and investors said.

ING said in February it had 4.8 billion euros of direct exposure to oil firms, less than 1 percent of its customer loans, providing some reassurance that the risks were manageable.

Natixis last week took no oil-related provision for the last quarter but forecast potential losses worth $250 million in worst-case stress tests. BNP Paribas also indicated no further provisioning was needed.

It will probably take time to figure out the extent of the damage.

Bernstein analysts Nick Green and Johan De Mulder say crunch time is still a few months away. Payments due on high-yield debt will peak above $11 billion in the second and fourth quarter of 2017 and at $8 billion in the second quarter of 2016. They estimate losses at European and UK banks of $13 billion and $5 billion respectively on their high-yield energy loans.

"Low oil prices aren't going away, and the risk is that we'll have … a slow burn of revelations and it will take a while for the full picture to emerge," IG strategist Chris Weston said.

($1 = 0.8988 euros)

(Reporting by Danilo Masoni; Editing by Sudip Kar-Gupta, Larry King)

By Danilo Masoni and Alistair Smout