While the crisis started in the U.S., Geithner said its damage has spread widely with serious challenges facing much of the globe.
"Never before in modern times has so much of the world been simultaneously hit by a confluence of economic and financial turmoil such as we are now living through," Geithner said in remarks to the Economic Club of Washington.
Geithner cited the International Monetary Fund's new economic forecast Wednesday that projected global economic output will this year, the first decline in more than six decades. Compared with a normal global growth rate of 4 percent, the lost output could amount to as much as $3 trillion to $4 trillion, he said.
Still, there are tentative signs the severity of the downturn was beginning to moderate. Some measures of consumer spending and industrial output had started to stabilize and financial conditions had improved "modestly," Geithner said.
It also was encouraging that the U.S. and other countries had responded with unprecedented speed and force to deal with the crisis, but going forward it will be critical for countries not to relent in their efforts to boost economic growth and stabilize their financial systems, he said.
"The actions now in place and in the pipeline offer the strongest basis for confidence that we will begin to lay the foundation for global recovery," Geithner said.
The by the Group of 20 major industrial countries and developing nations in London on April 2 offered a solid basis for support, and finance ministers meeting this weekend would be seeking to build on those commitments, he said.
Following his speech, Geithner avoided directly answering a question about the chances the government will be able to recoup the $182 billion in support it has provided to troubled insurance giant American International Group Inc.
Geithner called AIG an "extraordinary challenge," but said that the government-installed managers were working hard to "help improve the odds that the taxpayer is repaid."