The comment from Taro Aso is the latest sign that Japan and China — the biggest foreign creditors to the United States — are increasingly worried that the U.S. government shutdown and the standoff over the debt ceiling could wreak havoc on their trillions of dollars of investments in U.S. Treasury bonds.

"The U.S. must avoid a situation where it cannot pay (for its debt) and its triple-A ranking plunges all of a sudden," Aso told reporters following a cabinet meeting.

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"The U.S. must be fully aware that if that happens the U.S. would fall into fiscal crisis," he said.

Japanese officials held several emergency telephone conferences with U.S. Treasury Department officials on Monday, Japan's Nikkei newspaper reported, citing unnamed sources.

However, a senior Japanese government official shrugged off the report, suggesting instead that the subject had only been discussed as part of regular contact between the two countries.

On Monday, Chinese Vice Finance Minister Zhu Guangyao said Beijing had been in touch with Washington over the standoff, in which House Republicans have refused to increase the $16.7 trillion debt ceiling as they seek changes in President Barack Obama's signature healthcare law.

Unless Congress raises the debt ceiling, the United States would be left on the edge of an unprecedented default, the Treasury has warned.

The political standoff is in its second week, with much of the U.S. federal government closed and no signs of a breakthrough, although some glimmers of hope emerged on Monday as Obama said he would accept a short-term increase in the nation's borrowing authority to avoid a default.

TRILLIONS AT STAKE

As at July 31, China held $1.28 trillion in U.S. Treasury bonds and Japan held $1.14 trillion, Treasury Department data shows.

The last big confrontation over the debt ceiling, in August 2011, ended with an 11th-hour agreement under pressure from shaken markets and warnings of an economic catastrophe if a default were allowed to happen.

China is "naturally concerned about developments in the U.S. fiscal cliff," Zhu told reporters, saying it was Washington's "responsibility" to avoid a debt crisis and ensure the safety of Chinese investments.

Japan has previously expressed its concerns in diplomatic terms. Aso and Chief Cabinet Secretary Yoshihide Suga both said last week that the fiscal standoff was essentially a U.S. domestic problem.

But Aso added the shutdown could push up the yen against the dollar - a concern for Japan's export-reliant economy, which has benefited from a yen decline since Prime Minister Shinzo Abe won election in December on a reflationist policy platform.

Should the U.S. default on its debt, which the Treasury Department says could happen as soon as Oct. 17, "there would be a large international impact," Aso said last week. "If there is no prompt resolution, various impacts will emerge."

The yen has been rising this month as investors shed risk and seek the perceived safe haven of the Japanese currency. The dollar slipped on Tuesday to a two-month low of 96.55 yen.

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