The Senate's Democratic leadership is shielding US President Barack Obama from potentially embarrassing fallout by refusing to vote on new sanctions against Iran, the chamber's top Republican warned Wednesday.

Lawmakers from both parties are keen to expand economic penalties on Iran, which is in the midst of negotiations with world powers over its nuclear program.

One bipartisan proposal that Obama opposes would trigger new sanctions if last month's interim deal rolling back parts of Iran's nuclear program does not lead to a full-fledged agreement in six months.

Top Republican Senator Mitch McConnell lambasted Senate Majority Leader Harry Reid for ramming through a defense spending bill with no opportunity to add amendments, a move that avoids debate on Iran sanctions while the sensitive nuclear negotiations are ongoing.

Reid "won't allow a robust amendment process because he can't stomach a vote on Iran sanctions," McConnell said on the Senate floor.

"He knows the administration would lose that vote decisively, and he knows that many members of his own caucus would vote alongside Republicans to strengthen those sanctions," McConnell added.

"So rather than allow a democratic vote that might embarrass the administration, the majority leader simply won't permit that vote to happen."

McConnell essentially accused Obama and his top allies in the Senate of seeking to diminish Congress's role in determining national security policy on critical issues like Iran and the ongoing civil war in Syria.

Secretary of State John Kerry has made numerous trips to Capitol Hill urging lawmakers to hold fire on sanctions in order to give negotiations a chance.

The effort appears to have worked, with leaders of the Senate Banking Committee, tasked with compiling new sanctions legislation, announcing last week they would not introduce such a bill in the near future.

Asked if he would move to introduce sanctions in January, Reid said "no," but then qualified his answer.

"Well, let's wait and see. We have a lot to do in January," Reid said Tuesday.

"We'll see what Secretary Kerry comes up with for progress on the deliberations during that period of time."

In a bid to stand tough, the administration expanded its sanctions blacklist against Tehran last week, triggering a walkout by Iranians in the midst of four days of talks in Vienna.

Negotiations were set to resume Thursday in Geneva.