US President Barack Obama and British Prime Minister David Cameron have spoken about their expectations for the next round of negotiations on Iran's nuclear programme, as part of a phone conversation about several security issues in the Middle East, the White House said.

"On Iran, the president and prime minister reiterated their support for the P5+1's unified proposal and discussed their expectations for the next round of talks," the White House said on Tuesday.

Iran and the six world powers - the US, Britain, Russia, France, China and Germany - came close to a preliminary nuclear agreement at the weekend during talks in Geneva and decided to resume negotiations on November 20 in their attempt to defuse a decade-old standoff.

Israel and some Western governments fear Iran is using its nuclear programme as a covert means to develop weapons, a charge that Tehran denies.

Tightening sanctions

The White House warned US lawmakers on Tuesday that tightening sanctions on Iran could box America into a "march to war" and derail a diplomatic push to limit Tehran's atomic programme.

The warning marked a significant toughening of President Barack Obama's stance towards Congress. 

"The American people do not want a march to war," White House spokesman Jay Carney told reporters.

US Secretary of State John Kerry, who will hold a closed-door briefing on Iran at the Senate Banking Committee on Wednesday, wants a "temporary pause" in the imposition of additional sanctions on Tehran by US lawmakers to give diplomacy a chance.

"Having the negotiation does not mean giving up anything,” Kerry told reporters.

“The time to oppose it is when you see what it is, not to oppose the effort to find out what is possible,”he said.

Fresh from the talks, Kerry believes it would be a "mistake" for the US Congress to impose additional sanctions on Iran now amid negotiations with Tehran over its nuclear programme, State Department spokeswoman Jen Psaki said.

Source: Agencies