Five of the six world powers negotiating with Iran over its nuclear program have stepped back, leaving Washington to hammer out a deal with Tehran, a key US lawmaker said Tuesday.

"It's evident that these negotiations are really not P5+1 negotiations any more," Senate Foreign Relations Committee Chairman Bob Corker said as he emerged from a closed-door briefing by Obama administration officials on the status of nuclear talks with Iran.

"It's really more of a bilateral negotiation between the United States and Iran."

The five permanent UN Security Council members -- Britain, China, France, Russia and the United States -- plus Germany have undertaken years-long talks with Iran in a bid to halt the Islamic republic's nuclear drive.

Several rounds of sanctions have been imposed on Iran, cutting deeply into the country's economy.

Under an interim agreement reached in November 2013, Iran has diluted its stock of fissile materials from 20 percent enriched uranium to five percent in exchange for limited sanctions relief.

But two deadlines for a permanent agreement have already been missed, requiring the talks to be extended.

President Barack Obama met with German Chancellor Angela Merkel at the White House on Monday, and Obama said he saw no reason to further extend the current deadlines.

The present issue, Obama said, was "does Iran have the political will and the desire to get a deal done?"

With an end-March deadline for a political agreement approaching, and a final deal confirming technical details required by June 30, Corker said the key players are now essentially Washington and Tehran.

"I was in Munich this weekend (for an international security conference) and was very aware that this was becoming more of a one-on-one negotiation," the Senate Republican told reporters.

Corker and the Democrat he replaced as committee chairman, Senator Robert Menendez, left the latest briefing expressing concern about the administration basing negotiations on the need to maintain Iran's potential nuclear weapons "breakout" time to at least one year.

"One of my major concerns all along that is becoming more crystal clear to me, is that we are, instead of preventing proliferation, we are managing proliferation," Menendez said.

Having Iran just one year away from building a bomb would be "a different world and a far more challenging world," he added.

Iran president: world must 'seize opportunity' of nuclear deal
Tehran (AFP) Feb 10, 2015 - Iran's President Hassan Rouhani said Tuesday that world powers must "seize the opportunity" of a landmark nuclear deal, insisting Tehran had taken the "necessary steps" for an accord.

Rouhani's remarks appeared to be a response to US President Barack Obama, who on Monday said: "The issues now are -- does Iran have the political will and the desire to get a deal done?"

Speaking in Tehran, Rouhani said that although gaps remain between Iran and the P5+1 powers -- Britain, China, France, Russia and the United States plus Germany -- it was up to them to close a deal.

"Right now good progress has been made although we are some distance away from the final agreement," he said during a meeting with India's national security adviser, Ajit Doval.

"Iran has taken necessary steps and now it's the other side's turn to seize the opportunity," he added.

Two deadlines for a permanent agreement on Iran's controversial nuclear programme have already been missed, requiring the talks to be extended.

Negotiators are now working toward the political outline of a deal by March 31, with the cut-off point for the technical details of a comprehensive accord by June 30.

Disagreements centre on the extent of nuclear activities Iran will be allowed to continue and the timetable for the lifting of sanctions.

At a speech later Tuesday to mark the 36th anniversary of the Islamic revolution, Rouhani told foreign diplomats Iran still believed in a "win-win solution" in the talks.

"In recent months we have shown the flexibility necessary to resolve this political issue," he said. "We hope that the other negotiating party can show more than before. If so... in a short time the disputes can be resolved."

Iran's supreme leader Ayatollah Ali Khamenei, whose authority overrides Rouhani and as such he will have the final word on the nuclear issue, said Sunday he would rather see the talks fail than reach a "bad deal".

"The Americans keep reiterating that it's better to have no deal than a bad one. I fully agree with that," Khamenei said.

Both the United States and Iran have said in recent days they are against further time being added to negotiations.

In his remarks Monday, Obama said "I don't see a further extension being useful if they have not agreed to the basic formulation and the bottom line that the world requires to have confidence that they're not pursuing a nuclear weapon."

Iran denies seeking an atomic bomb and says its nuclear programme is for peaceful energy purposes only.