Ben Bernanke delivers a speech to the London School of Economics in London, 13 Jan 2009

A top U.S. economic official says a nearly $800 billion stimulus package may not be enough to promote a lasting economic recovery.

U.S. Federal Reserve chairman Ben Bernanke says the plan proposed by President-elect Barack Obama should provide the economy with a "significant boost." But he also warns the government must take strong steps to reform the financial system.

During a speech at the London School of Economics Tuesday, Bernanke also warned that the timing and strength of a global economic recovery are still "highly uncertain."

He said the financial crisis has shown that the world is too interconnected for nations to "go it alone" in economic, fiscal and regulatory policies.

Meanwhile, the U.S. government says the country's trade deficit plunged to its lowest level in five years in November as oil prices fell and consumers cut back on spending.

The government report says U.S. imports fell by 12 percent, with imports from China dropping by the largest amount on record.

Some information for this report was provided by AP and Reuters.