The United States is conferring with Israel about new sanctions planned against Iran should international negotiations this month fail to curb the Islamic Republic's nuclear programme, a U.S. official said on Monday.

The comment offered a strong hint that Washington is continuing to apply the brakes on any plan by Israel to attack Iranian nuclear facilities preemptively.

Israel has signalled increasing impatience with the lack of progress towards circumscribing the nuclear programme during the negotiations, involving Iran, the United States and five other world powers. The third round of talks will be hosted by Russia on June 18-19.

"If we don't get a breakthrough in Moscow there is no question we will continue to ratchet up the pressure," David Cohen, U.S. Treasury undersecretary for terrorism and financial intelligence, told Haaretz newspaper during a visit to Israel.

The United States and European Union have already made clear they will stiffen sanctions should Iran pursue uranium enrichment, a process that can yield fuel for warheads though it insists the objective is civilian energy and medical isotopes.

Cohen stressed the depth of the U.S.-Israeli partnership.

"We have today and over the past years had very close cooperation with the Israeli government across a range of our sanctions programmes," he said. "They are creative. They are supportive and we will continue to consult with the Israelis."

Cohen made similar comments to Army Radio, a major Israeli broadcaster, during his 36-hour visit, when he was to meet with Prime Minister Benjamin Netanyahu's senior security staff.

In a speech last week, Netanyahu said world powers must both beef up sanctions and demand an immediate end to all uranium enrichment by Iran, whose mid-level 20 percent purification has been the focus of earlier negotiations.

Israel is reputed to have the region's only atomic arsenal and many international experts, including the top U.S. military officer, General Martin Dempsey, have voiced doubt in the ability of its conventional forces to deliver lasting damage to Iran's distant, dispersed and well-defended nuclear facilities.

The Israelis have hinted that delaying Iran's progress could justify a unilateral strike. Ensuing Iranian reprisals would risk drawing in the United States, which has not ruled out force against Tehran but is loath to launch a new military campaign in the Muslim world. (Writing by Dan Williams; Editing by John Stonestreet)