Earlier Sunday, the Obama adminstration announced that Mr. Geithner would be staying on as secretary, a move whose timing appeared intended to reassure nervous investors.

As officials huddled to discuss strategy, traders and investors around the world also prepared for the fallout from the downgrade of the United States’ credit rating, as markets in Asia neared their opening Monday. Although many experts said the impact in the bond market would not be as stark as first feared, the ruling by Standard & Poor’s on Friday has already unnerved global stock markets.

Shares across the Middle East fell sharply Sunday, with stocks in Israel falling 7 percent, the steepest daily fall in more than a decade. United States stock futures were lower, as well.

Stock markets in the Asia-Pacific region fell Monday, with the Nikkei index in Japan down 1.3 percent at midday, and the price of gold — considered a safe haven at times of uncertainty — jumped to yet another nominal record high, to more than $1,688, underscoring the lingering anxiety. Futures on the Standard & Poor’s 500 were 1.8 percent lower, and the price of oil sagged $3 a barrel.

After the G-7 conference call, Yoshihiko Noda, the Japanese finance minister, told reporters that global markets’ trust in both United States Treasuries and the dollar remained “unshaken.”

With signs of slowing growth in the United States and Europe, and government budgets and central bank balance sheets already stretched to the limit, the options for policy makers are dwindling. “None of them have a lot of things to do to alleviate the crisis,” Carl B. Weinberg, chief economist of High Frequency Economics in Valhalla, N.Y., said Sunday. “Fiscal stimulus is not an option right now.”

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In addition, it was not clear whether Washington would be able to break out of its partisan stalemate and produce in the next few months the kind of overarching deficit reduction that credit agencies and the markets will most likely demand.

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“I just keep asking for the sake of the economy: can’t we wait on the things that we’re going to yell at each other about and start on the things that we agree on,” Austan Goolsbee , chairman of the White House Council of Economic Advisers , said on NBC ’s “Meet the Press.”

In an interview with CNBC on Sunday, Mr. Geithner emphasized how critical it was for lawmakers to put aside their differences. “Congress ultimately owns the credit rating of the United States,” he said. “They have the power of the purse of the Constitution. And they’re going to have a chance now to earn back the confidence of the investors around the world.”

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The country, he said, “is much stronger than Washington.”

This week the focus may shift to policy makers at the Federal Reserve, who meet on Tuesday, but their options are narrower than they have been at other moments of turbulence in global markets. Short-term interest rates are already near zero, and cannot be lowered further, while long-term rates are also at historic lows.

Another series of huge purchases of government bonds and other assets — so-called quantitative easing — remains an option, but two earlier rounds have done little to bolster consumer confidence or economic growth.

Some analysts said that Italy, which is fast losing investor confidence, posed a threat to the financial system too great for any one economic superpower to cope with alone, and that a coordinated effort was needed. “They just can’t allow the Italian economy to go down the tubes,” Uri Dadush, a senior associate at the Carnegie Endowment for International Peace, said Sunday. “It would be a Lehman -type situation.” He was referring to the collapse of the investment bank Lehman Brothers in September 2008, which helped to touch off the global financial crisis.

Mr. Dadush put the cost of a bailout of Italy, the third-largest economy in the euro zone, at $1.4 trillion, with Spain requiring $700 billion. Those sums are far beyond the capability of the Continent’s current rescue fund. Mr. Weinberg and other analysts said that bond buying by the European Central Bank was likely to provide only temporary relief. The bank’s intervention in the much smaller market for Greek bonds failed to prevent a de facto default.

“This is just a Band-Aid to show the markets they are doing something,” Mr. Weinberg said.

European leaders nevertheless sought to offer their own reassurances to the markets. Late Sunday, the governments of France and Germany issued a joint statement promising to quickly enact the expanded bailout package for Greece that they agreed to last month. They also praised efforts by Spain and Italy to improve their finances and competitiveness.

The central bank intervened in European bond markets last week for the first time since March. But it appeared that it was buying relatively small amounts of Portuguese and Irish bonds.

The bank may have intended this to signal its resolve, but markets seemed to have interpreted the modest intervention as a sign of weakness, creating pressure for a bolder move. The bank’s vigorous response Sunday was a significant escalation of the milder intervention of the past.

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Markets are likely to quickly test how far the central bank will go to protect Italy and Spain. Even the bank would be hard-pressed to buy enough bonds to hold down yields on Spanish and Italian debt in the long term, and prevent the countries’ borrowing costs from reaching levels that would eventually prove ruinous, analysts said.

European governments have agreed to use their joint bailout fund, the European Financial Stability Facility, to buy government bonds on the secondary market. But Germany will be reluctant to risk its own credit rating by committing vast funds to Spain and Italy, a move that would be politically unpopular as well.

In any event, it will be several months before the bailout fund has the resources and legal authority to begin buying bonds.