The Standard & Poor's rating agency has affirmed the United States' long-term debt rating at AA+, but noted the world's largest economy faced uncertain fiscal policy.

The rating, one notch below the top grade, is justified by the "resilient" US economy, flexible monetary policy and America's unique position as the issuer of the global currency of reserve, S&P said in a statement.

The US lost its AAA rating in 2011, following battles among lawmakers in Washington over whether to lift caps on US sovereign borrowing, raising the likelihood of a US default.

S&P noted that another deadline for raising the debt ceiling is looming.

While Congress is expected to raise or suspend the debt limit albeit "potentially with heated discussion," S&P said the debate "weighs on the economy."

S&P noted the high level of US government debt, and said uncertainty about the future of economic policy constrains the US rating, even while it also affirmed a stable outlook.