WASHINGTON, Jan. 19 (Xinhua) -- Fitch, one of the top three world ratings agencies, said on Friday its top AAA debt rating for the U.S. wouldn't be directly affected by a possible federal government shutdown.

The U.S. Senate was racing to avert a government shutdown on Friday by passing a short-term budget extension. Mick Mulvaney, White House Office of Management and Budget Director, said Friday the government is preparing for a shutdown.

"Partial federal government shutdowns have occurred in the past and this shutdown does not have a direct impact on the sovereign's 'AAA'/Stable rating," Fitch said in a statement.

Such stoppages have happened 18 times in history, most recently in 2013. The most famous government dysfunction happened in 1995, and the month-long shutdown cost the U.S. economy 1.4 billion dollars.

"Its main implication for the United States' sovereign creditworthiness would depend on whether it foreshadowed a further destabilization of U.S. budget policymaking, or brinkmanship over the federal debt limit," Fitch said.