LONDON — Federal Reserve Chairman Ben Bernanke said Tuesday the stimulus package being crafted by President-elect Barack Obama and Congress could provide a "significant boost" to the sinking economy. But he warned that such a recovery won't last unless other steps are taken to stabilize the shaky financial system.

Although Bernanke has previously endorsed the notion for a fresh round of government stimulus to lift the country out of a recession, it marked the first time the Fed chief has referenced the roughly $800 billion recovery plan now being worked on by Obama, who takes office next week. Obama envisions a blend of tax cuts and increased government spending, including on big public works projects, to make up the stimulus plan.

Bernanke, who didn't weigh in on the details of the evolving package, made clear that such a recovery plan was needed as part of a broader, multi-pronged government response to combat the worst financial crisis to hit the U.S. and the global economy since the 1930s.

"The incoming administration and the Congress are currently discussing a substantial fiscal package that, if enacted, could provide a significant boost to economic activity," Bernanke said in a speech to the London School of Economics.

"In my view, however, fiscal actions are unlikely to promote a lasting recovery unless they are accompanied by strong measures to further stabilize and strengthen the financial system," he warned. "History demonstrates conclusively that a modern economy cannot grow if its financial system is not operating effectively."

To help on that front, the Fed is loaning out billions to financial companies and buying mounds of companies' debt to help bust through the debilitating credit clog. And the Treasury Department is overseeing a $700 financial bailout program that has pledged to inject $250 billion into banks in return for partial government ownership. Some money from the bailout pot also is being used to guarantee against possible losses from risky assets held by Citigroup Inc.

Bernanke said "more capital injections and guarantees may become necessary" to stabilize financial markets and spur more lending. If Obama's incoming Treasury secretary Timothy Geithner decides to remove toxic assets from financial institutions' balance sheets — the original but abandoned strategy under the $700 billion bailout — Bernanke suggested some options to do that.