Congress buried the specter of a U.S. debt default by finally passing a deficit-cutting package on Tuesday, but uncertainties lingered over a possible painful downgrade of the top-notch American credit rating.

President Barack Obama welcomed as "an important first step" the hard-won deficit-cutting compromise to lift the government's $14.3 trillion debt ceiling after it was approved by the U.S. Senate with a vote of 74 to 26.

But, signaling possible tough political battles ahead over spending cuts and tax reform, Obama said the sacrifices required to reduce the U.S. deficit needed to be fairly shared in U.S. society, including by the wealthiest.

"Everyone is going to have to chip in, that's only fair," the president said in an address from the White House Rose Garden after the Senate approved the debt limit deal.

Final Congress approval came just hours before the Treasury's authority to borrow funds was to run out.

Obama, who will seek a second term next year, was expected to immediately sign the deal into law on Tuesday.

His signature would draw a line under months of rancorous partisan squabbling over debt and deficit strategy that had threatened chaos in global financial markets and dented America's stature as the world's economic superpower.

There was little suspense about the outcome of the vote in the Democratic-controlled Senate.

The bill overcame its biggest hurdle late on Monday when the Republican-led House of Representatives passed the $2.1 trillion deficit-reduction plan despite some resistance from recalcitrant Tea Party conservatives and disappointed liberal Democrats.

Uncertainty remained, however, over whether the budget deal goes far enough in reining in deficits to satisfy major ratings agencies, which have threatened to downgrade the United States' AAA credit rating. Such a move would raise borrowing costs and act as another drag on the stumbling economy.

Ratings agency Standard and Poor's said in mid-July there was a 50-50 chance it would cut U.S. ratings in the next three months if lawmakers failed to craft a meaningful deficit-cutting plan.

FITCH: U.S. DEFAULT RISK "EXTREMELY LOW"

Another agency, Fitch Ratings, on Tuesday called the agreement reached an "important first step" but said it was not the end of a process to put in place a credible deficit plan.

It added that the United States, like Europe, must also confront tough choices on tax and spending against a weak economic backdrop if the budget deficit and government debt was

to be cut to safer levels over the medium term.

Treasury Secretary Timothy Geithner said earlier he expected the ratings agencies to take a "careful look" at the situation but he was not sure whether the United States would be spared from a downgrade.

"I don't know. It's hard to tell," he told ABC News.

Initial relief in financial markets over an end to the gridlock quickly turned to concern on Tuesday about risk of a U.S. ratings cut as well concerns based on recent economic data that growth could remain subdued. U.S. stocks dropped even as Congress approved the deal.

The plan, which lifts the debt ceiling enough to last beyond the November 2012 elections, calls for $2.1 trillion in spending cuts spread over 10 years and creates a congressional committee to recommend a deficit-reduction package by late November.

That appears to fall short of rating agency S&P's previous assertion that $4 trillion in deficit-reduction measures would be needed to avoid a downgrade by showing that Washington was putting the country's finances in order.