Top US diplomats will this week make a "strong case" to lawmakers to hold off on fresh Iran sanctions, hoping to avoid an ugly political showdown which could scupper nuclear talks.

Acting Deputy Secretary Wendy Sherman, who has led the tense, secretive negotiations for months, will be at the vanguard of an administration drive to persuade Congress not to push ahead with draft legislation which would slap even more punitive sanctions on Iran.

Hawkish lawmakers are eager to pass new sanctions through Congress aiming to prod the Islamic republic into a lasting deal that prevents Iranian leaders from developing a nuclear weapon.

But experts say such a move would jeopardize months of delicate diplomacy.

Sherman will brief lawmakers behind closed doors on Thursday, a State Department spokeswoman said Tuesday, ahead of a flurry of consultations over the next two weeks.

US Secretary of State John Kerry and his global partners in the P5+1 group last week missed a second deadline to nail down a comprehensive deal with Iran to rein in its suspect nuclear program.

Instead, the seven nations agreed to extend an existing interim deal for a further seven months until June 30, with the hopes of drawing up a framework accord by March.

Under the interim deal, Iran receives limited access to its frozen oil revenues in exchange for freezing its nuclear enrichment.

Tehran has long denied seeking an atomic bomb saying its program is purely for civilian energy purposes. But Western powers remain skeptical.

"We will make a very strong case to them why now is not the time for new sanctions," deputy State Department spokeswoman Marie Harf told reporters of Sherman's planned meetings in Congress.

"We'll make it publicly. We'll make it privately. We'll make it at all levels because we believe very strongly in it."

Officials would spell out to skeptical lawmakers the progress that has been made over the past months of negotiations, Harf said.

"We can't, obviously, share every single detail, to preserve the integrity of the negotiations," she said, but insisted lawmakers should "keep in investing in diplomacy."

Experts have warned Tehran would be infuriated by new sanctions which would also risk eroding international support for the economic embargo and alienate other allies such as Turkey, India, China.

"Look, we know one of the reasons Iran is at the negotiating table today, if not the biggest reason, is because of the sanctions that Congress put in place, sanctions the secretary of state wholeheartedly supported," Harf said. "So sanctions played a key role here."

She stressed again however that now is "not the time for new sanctions."