LAUSANNE, Switzerland - The United States and Iran are drafting elements of a nuclear deal that commits Tehran to a 40 percent cut in the number of machines it could use to make an atomic bomb, officials told the Associated Press on Thursday. In return, the Iranians would get quick relief from some crippling economic sanctions and a partial lift of a U.N. embargo on conventional arms.

Agreement on Iran's uranium enrichment program could signal a breakthrough for a larger deal aimed at containing the Islamic Republic's nuclear activities.

The sides are racing to meet a March 31 deadline for a framework pact and a full agreement by the end of June - even as Congress keeps up pressure on the Obama administration to avoid any agreement leaving Iran with an avenue to become a nuclear power.

Officials said the tentative deal imposes at least a decade of new limits on the number of centrifuges Iran can operate to enrich uranium, a process that can lead to weapons-grade material. The sides are zeroing in on a cap of 6,000 centrifuges, officials said.

That's also fewer than the 10,000 such machines Tehran now runs, yet substantially more than the 500 to 1,500 that Washington originally wanted as a ceiling.

But U.S. officials insist the focus on centrifuge numbers alone misses the point. Combined with other restrictions on enrichment levels and the types of centrifuges Iran can use, Washington believes it can extend the time Tehran would need to produce a nuclear weapon to at least a year.

It's unclear how complete the draft is. Iran's underground enrichment plant remains a problem, officials said, with Washington demanding the facility be repurposed and Tehran insisting it be able to run hundreds of centrifuges there.

Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif met for a fourth day Thursday. "We are pushing some tough issues," Kerry said after a meeting. "But we made progress."