Leading credit rating agency Fitch Ratings has lowered the rating of US government bonds.

Fitch on Tuesday announced it had downgraded the United States' Long-Term Foreign-Currency Issuer Default Rating from "AAA" to "AA+."

The firm's move reflects expected US fiscal deterioration over the next three years and the political turmoil that has manifested in repeated standoffs over the government debt limit.

The US rating was last downgraded in August 2011 when Standard & Poor's -- now S&P Global Ratings -- lowered it by one notch.

US Treasury Secretary Janet Yellen on Tuesday issued a statement saying she strongly disagreed with Fitch Ratings' decision. She called it "arbitrary and based on outdated data."

Yellen said, "Fitch's decision does not change what Americans, investors and people all around the world already know: that Treasury securities remain the world's preeminent safe and liquid asset, and that the American economy is fundamentally strong."

She noted that she and President Joe Biden "are committed to fiscal sustainability."

Yellen added that the most recent debt limit legislation included over 1 trillion dollars in deficit reduction and improved the US' fiscal trajectory.