The nation's new intelligence chief warned Thursday that the global economic crisis is the most serious security peril facing the United States, threatening to topple governments, trigger waves of refugees and undermine the ability of the nation's allies to help in Afghanistan and elsewhere.

WASHINGTON — The nation’s new intelligence chief warned Thursday that the global economic crisis is the most serious security peril facing the United States, threatening to topple governments, trigger waves of refugees and undermine the ability of the nation’s allies to help in Afghanistan and elsewhere.

The economic collapse “already looms as the most serious one in decades, if not in centuries,” said Dennis Blair, director of national intelligence, in his first appearance before Congress as the top intelligence official in the Obama administration.

Blair’s focus on the economic meltdown represents a change from the testimony of his predecessors, who devoted most of their attention in the annual threat-assessment hearing to terrorism and the wars in Afghanistan and Iraq.

Blair’s conclusions figure to bolster President Obama’s case for swift action on a $789 billion stimulus package nearing final approval. “Time is probably our greatest threat,” Blair said. “The longer it takes for the recovery to begin, the greater the likelihood of serious damage to U.S. strategic interests.”

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Blair said one-quarter of the world’s nations have experienced low-level instability attributed to the economic downturn, including shifts in power. He cited anti-government demonstrations in Europe and Russia, and he warned that much of Latin America and the former Soviet satellite states lack sufficient cash to cope with the spreading crisis.

“The most likely potential fallout for U.S. interests will involve allies and friends not being able to fully meet their defense and humanitarian obligations,” Blair said.

The decline in oil prices in recent months might benefit consumers in the short term and “put the squeeze on the adventurism of producers like Iran and Venezuela,” Blair said.

But he warned it also could lead to a supply crunch if prolonged price drops lead to cuts or delays in investment in oil development and infrastructure.

Other crises in recent decades have tended to be concentrated by region, making it possible for affected countries to rebuild by focusing on shipping more products offshore.

But “countries will not be able to export their way out of this one because of the global nature” of the crisis, Blair said.

U.S. intelligence analysts fear there could be a backlash against U.S. efforts to promote free markets because the crisis was triggered by the United States.

Blair surveyed other security threats in his written testimony. The nation’s most dangerous enemy remains al-Qaida, but Blair said the terrorist network “is less capable and effective than it was a year ago” because of the toll that U.S. missile strikes and other measures have taken on al-Qaida’s sanctuary in Pakistan.