US Secretary of State John Kerry warned Congress Wednesday that passing new sanctions against Iran would scuttle diplomatic efforts to rein in Tehran's nuclear drive, although several lawmakers came away unconvinced.

"We now are negotiating, and the risk is that if Congress were to unilaterally move to raise sanctions, it could break faith with those negotiations and actually stop them and break them apart," Kerry said.

Washington's top diplomat spoke to reporters before beginning a closed-door meeting with members of a Senate banking panel, many of whom are skeptical of the White House's request for a freeze on new sanctions.

"The pitch was very unconvincing," Republican Senator Mark Kirk said afterward.

Fellow Republican Bob Corker said Kerry made an "emotional appeal" but did not provide much-sought-after details of the proposal that Washington and other western powers put on the table for Iran.

"I was very disappointed in the presentation. It lacked content," Corker said.

Democrat Tim Johnson, the Senate Banking Committee's chairman, merely said he was "undecided" about whether to proceed with new sanctions.

Kerry emerged arm in arm with Vice President Joe Biden, who joined his fellow former senator in a briefing with senior Democrats after Kerry's meeting with the banking panel, but neither of them spoke to reporters on the way out.

The House of Representatives has already passed legislation that toughens already-strict sanctions on Iran, whose economy by all accounts is reeling from the punitive action.

The banking panel is mulling new sanctions too, and some key members of President Barack Obama's own Democratic Party back a tougher stance despite the diplomatic opening.

"What we're asking everybody to do is calm down, look hard at what can be achieved and what the realities are," Kerry said.

"Let's give them a few weeks, see if it works," he said, adding that there was "unity" among the six powers negotiating with the Islamic republic.

The so-called P5+1 -- UN Security Council permanent members Britain, China, France, Russia and the United States, plus Germany -- has contemplated offering relief to Iran on some economic sanctions in exchange for nuclear concessions.

"If this doesn't work, we reserve the right to dial back up the sanctions," Kerry said.

In that event Kerry said he would return to Capitol Hill "asking for increased sanctions. And we always reserve the military option."

Washington and Western allies allege Iran is trying to develop a nuclear weapon, a charge Tehran denies.

Obama has vowed he will not allow Tehran to develop an atomic arsenal. But last week's Geneva talks between Iranian and western negotiators failed to reach an interim deal to halt its program.

Meanwhile Kerry warned that new sanctions could convince coalition partners to "bolt" from the negotiations because the United States was not a good-faith partner.

Senate Republicans and Democrats alike bristled when the White House warned Tuesday that toughening sanctions could trigger a "march to war."

The administration's remarks marked a significant hardening of Obama's stance towards Congress on sanctions as Washington prepares to resume talks with Iran on November 20.

As he entered the meeting, Kerry addressed criticism that negotiations failed in Geneva, saying Iran would have jumped at the interim deal if it was to their benefit.

"We have a pause because it's a tough proposal, and people need to think about it, obviously," Kerry said.