At a central banking conference before a meeting here of the Group of 20, Mr. Bernanke said that while the global financial crisis was receding, ?capital flows are once again posing challenges for international macroeconomic and financial stability.? He added, ?They reflect the two-speed nature of the global recovery as emerging market growth far outstrips growth in advanced economies.?

Mr. Bernanke called on countries that export more than they import to allow their currencies to reflect its overall economic performance, and urged those with large trade deficits to increase their savings and put their fiscal policies on more sustainable path.

Mr. Bernanke did not mention China in his remarks, although they appeared to be a veiled reference to Beijing?s policy of keeping its currency, the renminbi, artificially low to gain a trade advantage. As Mr. Bernanke delivered his remarks, the Chinese central bank governor, Zhou Xiaochuan, appeared on the same stage.

The Fed chairman said countries that have been maintaining undervalued currencies have ?contributed to spending that?s unbalanced and unsustainable,? which, together with surging demand in fast-growing emerging markets, are fueling a worrisome rise in commodity prices.

Meanwhile, countries that have allowed market forces to determine the level of their money have been penalized by seeing their competitiveness erode, he said.

?Spillovers can go both ways,? Mr. Bernanke said. ?Resurgent demand in the emerging markets has contributed significantly to the sharp recent run-up in global commodity prices.?

Mr. Zhou later appeared to concede the point, to a degree. He noted that Chinese exporters complain that they would all go out of businesses if the government were to allow the renminbi to rise by 3 percent. ?But then that happens and then they survive,? he said. In fact, he added, ?they have room to improve and survive.?

Policy makers in advanced economies have been calling on China to stoke demand from within its own country to even out its trade dominance. Mr. Zhou said China was working on that. ?One day we?ll have a domestic market too, depending on price elasticity.? he said.

China could also shift from an export-dominated manufacturing economy to a more service-oriented one in order to meet domestic needs. ?But that would be slow, it would take 10 years at best,? Mr. Zhou said.

In a star-studded lineup from the world of economic policy makers, the central bank governors of Britain and France, as well as Jean-Claude Trichet, the governor of the European Central Bank, took turns underscoring their worry that the uneven nature of global growth could undermine the recovery.

Officials from the Group of 20 are gathering here Friday and Saturday, and are near agreement on a set of ?indicators? to help identify when economic and financial developments in some countries would pose problems for all the others.

Both China and Germany have batted down calls from the United States to cap current-account surpluses or deficits at a certain percentage of economic growth.

Christine Lagarde, the finance minister of France, which is presiding over the meeting this year, warned that if countries stuck to ?business as usual,? the world risked falling into a debt crisis.

?The more desirable scenario is that deficit countries curb and reduce debt, and surplus countries are prepared to boost their domestic demand,? she said. ?Where collective gains are possible, global growth would be reconciled by a reduction of imbalances,? she said.