Two top US senators agreed with the White House Thursday that Washington should not introduce new sanctions against Iran, warning they could "rupture" international unity against Tehran's nuclear program.

The comments by the top Democrat and Republican on the Senate Banking Committee virtually assured that no new sanctions legislation would pass Congress before the year-end break, although lawmakers could controversially introduce a new sanctions bill within the next week.

Senate Banking Committee chairman Tim Johnson and the committee's top Republican Michael Crapo appeared to support the current diplomatic pause sought by President Barack Obama to allow an interim agreement with Iran that halts its nuclear program to take hold.

"Let me be clear. I support strong sanctions, and authored many of the US sanctions currently in place," said Johnson, who has been in the spotlight over whether he would introduce a new sanctions regime this year, as several senators have sought.

"But I agree that the administration's request for a diplomatic pause is reasonable," Johnson told a hearing by the panel.

"A new round of US sanctions now could rupture the unity of the international coalition against Iran's nuclear program."

Johnson also made clear that he was already crafting new sanctions behind closed doors.

"I have negotiated a new bipartisan sanctions bill with my ranking member (Crapo) that could be finalized and moved quickly if Iran fails to comply with the terms of the first-step agreement in Geneva, or if negotiations collapse."

Johnson's fellow Senate Democrat Robert Menendez and Republican Mark Kirk also have been drafting a bill that they want to see passed sooner rather than later.

It would impose tough new economic sanctions on Iran if it renegs on the six month interim deal it struck last month with the P5+1 group -- the United States, Britain, China, France, Russia and Germany.

Menendez and Kirk have the support of Senator John McCain, who said Wednesday he hoped the new legislation could bypass committee consideration and be introduced directly to the Senate floor.

But on Thursday Crapo appeared focused on enforcing "existing" sanctions pressure.

"The United States must continue to vigorously enforce the existing core sanctions architecture and develop a sanctions plan of action in the event that negotiations do not produce the result diplomats want," Crapo told the hearing.

Two administration officials, including the Treasury's Under Secretary for Terrorism and Financial Intelligence David Cohen, warned that new congressional action could derail the agreement altogether.

"There'd be a real risk to the effectiveness of our sanctions regime if Congress were to pursue new sanctions legislation now," Cohen testified.