WASHINGTON – The International Monetary Fund says the global financial system faces more challenges than at any point since the 2008 financial crisis.

Europe’s debt crisis is spreading to its banks, which hold government debt and may be forced to pull back on lending to conserve cash, the lending organization said Wednesday.

The U.S. economy is being restrained by its depressed housing market. Many homeowners owe more on their mortgages than their homes are worth. That has limited their ability to spend and is holding back growth.

The IMF urged European banks to raise more capital to shore up their finances. If necessary, governments should provide it. And it said the U.S. government should help homeowners reduce their mortgage debt.

“Risks are elevated, and time is running out to tackle vulnerabilities that threaten the global financial system and the ongoing economic recovery,” the IMF said in its semi-annual Global Financial Stability report.

The crisis in Europe is entering a dangerous new phase. Fears are growing that Greece may default on its debts. That would destabilize other debt-laden European countries, such as Spain and Italy. It would also cause substantial losses at French, German and other banks.

European leaders should quickly implement a July agreement that provides the region’s bailout fund with more flexibility, the IMF said.

The agreement allows the bailout fund to purchase bonds issued by debt-strapped countries such as Greece and Ireland, and would make it easier for Europe to resolve its debt crisis.

And Europe’s larger banks, which hold substantial amounts of Greek and other troubled government bonds, should boost their capital reserves, the IMF said. That would protect them in case the bonds lose more of their value or Greece defaults on its debts.

The capital should come from private markets, the IMF said. But if that isn’t available, governments should provide it.

European banks face potential losses of $274 billion from shaky government bonds, and $410 billion if the risk of losses on loans to other banks were included. The IMF said the figure didn’t represent the amount of capital banks needed to raise.

In the U.S., about one-quarter of homeowners owe more on their homes than they are worth. Reducing that debt burden would improve consumer demand and support growth.

“Restoring confidence in the stability of the U.S. housing market is the key to bolstering the prospects for U.S. banks,” which have been hurt by slower growth, the report said.

But U.S. lawmakers are focused on cutting spending rather than enacting new programs to aid homeowners. And President Barack Obama didn’t include any new measures to bolster the depressed housing market in a jobs package he proposed earlier this month.