(Updates with Obama-Cameron phone call)

WASHINGTON, Nov 12 (Reuters) - Secretary of State John Kerry will tell U.S. lawmakers on Wednesday it would be a mistake for Congress to impose new sanctions on Iran now amid talks with Tehran over its nuclear program, the State Department said.

Kerry wants a "temporary pause" on new sanctions to allow diplomats from six world powers, including the United States, to negotiate with Iran and to test whether it may be possible to resolve a 10-year standoff over the Iranian nuclear program, department spokeswoman Jen Psaki told reporters.

Negotiations over the weekend between Iran and the major powers failed to reach an agreement on curbing the program, which Washington and its allies believe may be designed to develop nuclear weapons - something Tehran denies.

Psaki said Kerry will counsel a go-slow approach on Wednesday when he briefs the Senate Banking Committee, a key congressional panel in drafting Iran sanctions legislation.

"The secretary will be clear that putting new sanctions in place would be a mistake while we are still determining if there is a diplomatic path forward," Psaki said at her daily briefing.

"What we are asking for right now is a pause, a temporary pause, in sanctions," she added. "This is about ensuring that our legislative strategy and our negotiating strategy are running hand in hand."

Negotiations between Iran and the six major powers that make up the P5+1 group - Britain, China, France, Germany, Russia and the United States - broke off without an agreement and officials are scheduled to return to Geneva for a second round of talks on Nov. 20.

President Barack Obama spoke by phone on Tuesday with British Prime Minister David Cameron about the negotiations.

"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 in a statement.

During the first round of negotiations, French Foreign Minister Laurent Fabius said Paris could not accept a "fool's game" - in other words, one-sided concessions to Iran.

Diplomats from other Western nations at first reacted angrily, accusing the French of trying to upstage the other powers and causing unnecessary trouble for the talks.

On Monday though, Kerry said the major powers were unified on Saturday when they presented a proposal to the Iranians, and suggested it was the Iranians who were unable to accept the proposal without consulting with Tehran.

"The French signed off on it, we signed off on it, and everybody agreed it was a fair proposal. There was unity. But Iran couldn't take it at that particular moment. They weren't able to accept that particular thing," Kerry told reporters.