President Barack Obama on Wednesday threatened to veto a Republican tax plan as talks to avert a fiscal crisis by the end of the year turned sour despite recent progress.

The White House rejected the Republicans' proposal, known as "Plan B," because it does not do enough to raise taxes on the wealthy or address spending cuts needed to tame the country's $16 trillion debt.

"The congressional Republican "Plan B" legislation continues large tax cuts for the very wealthiest individuals," White House Communications Director Dan Pfeiffer said in a statement.

House of Representatives Speaker John Boehner's office used strong words to slam Obama for opposing the plan, which would mean higher taxes for the first $1 million of income and above.

"The White House's opposition to a backup plan ... is growing more bizarre and irrational by the day," said Brendan Buck, a spokesman for Boehner.

Senior administration officials said Republican plans to vote their "Plan B" this week in the House has brought negotiations to a standstill.

Still, both sides appeared confident that a deal remained within reach, with the White House urging Republican leaders to work to find a deal "today" and the top Republican in the Senate saying resolution could come by the end of the week.

Obama and Boehner, the top Republican in Congress, have each offered big concessions in talks.

They want to reach a deal soon to avert automatic tax hikes and spending cuts set to begin next month. Known as the "fiscal cliff," the measures could trigger another recession.

"There's still enough time for us to finish all of our work before this weekend, if we're all willing to stay late and work hard," said Senate Republican Leader Mitch McConnell.

Republicans have increased pressure on the Democrats by vowing to vote in the House this week on their "Plan B" backup measure that would largely disregard the progress made so far in negotiations.

The plan is also a way of providing political cover for Republican lawmakers who could afterward tell constituents back home that at least they voted for it and, in doing so, did their best to try to block Obama's agenda.

CLOSE BUT NO DEAL

After progress made over the last week that has bridged ideological differences, the Obama-Boehner talks are focused increasingly on narrower disagreements over numbers.

"What separates us is probably a few hundred billion dollars," Obama said at a news conference.

This has soothed nervous financial markets.

U.S. stocks were little changed early on Wednesday.

Still, any deal by Obama and the Republican leadership would need the support of their parties' rank and file.

Many Democrats dislike the president's offer to reduce benefits to seniors, although some political allies of Obama have given signs they feel they could swallow this concession.

"I don't like these particular changes," said Democratic Representative Chris Van Hollen, a member of the House leadership from Maryland. But he added: "What people are seeing is the president willing to compromise in order to get things done."

Boehner has also conceded to Obama's demand that taxes rise for the rich, although the two sides have yet to define what counts as wealthy. Obama said he was willing to take steps that make some Democrats unhappy, and was puzzled Republicans have not taken his offer.