WASHINGTON - The United States held out the possibility on Thursday of giving Iran some short-term sanctions relief in return for concrete steps to slow uranium enrichment and shed light on its nuclear program.

Under Secretary of State Wendy Sherman, the lead U.S. negotiator with Iran, also urged lawmakers to hold off imposing additional sanctions against Iran before October 15-16 when six major powers - Britain, China, France, Germany, Russia and the United States - will meet Iranian officials to negotiate over Tehran's nuclear program.

The scheduled talks follow this year's election of Iranian President Hassan Rouhani, a centrist who has made overtures to the West and spoke last week by telephone with U.S. President Barack Obama in the highest-level contact between the two countries since 1979.

In testimony to Congress, Sherman held out the possibility of sanctions relief for Iran but made clear that the United States expects concrete actions from Tehran before this could happen. She said all U.S. concerns about Iran's nuclear program must be addressed before the core sanctions could be removed.

"We will be looking for specific steps by Iran that address core issues, including but not limited to, the pace and scope of its enrichment program, the transparency of its overall nuclear program and stockpiles of enriched uranium," Sherman told the Senate Foreign Relations Committee.

"The Iranians in return will doubtless be seeking some relief from comprehensive international sanctions that are now in place," she added. "Only concrete ... and verifiable steps can offer a path to sanctions relief."

Senate Foreign Relations Committee Chairman Robert Menendez, a Democrat, voiced concern about early sanctions relief, saying this could undermine international support for the economic penalties that would then be very hard to restore.

Sherman said the fundamental, major sanctions - which she did not name  - should remain in place until all U.S. concerns about Iran's nuclear program are addressed, but suggested some openness to partial sanctions relief as negotiations proceed.

Because of the technical complexity of securing agreement on Iran's nuclear program, it would be useful to find a way to halt its progress to provide time for the negotiations, she said.

"What we are thinking through is, what is it that would give us some confidence today, would put some time on the clock, stop their nuclear program from moving forward, while we get to that comprehensive agreement that would allow the full sanctions relief they are looking for," Sherman said.

"There may be some elements that we can do initially if they take verifiable, concrete actions that will put time on the clock that are reversible, or in fact don't go to any of the key sanctions that have brought them to the table."

Together with its allies, the United States, which broke diplomatic ties with Iran in 1980 after the Islamic revolution, suspects that Iran is using its civilian nuclear program as a cover to develop atomic weapons.

Iran denies this, saying its program is for solely civilian and peaceful purposes.

The United States and its allies have imposed extensive sanctions against Iran, including a U.S. law that forced buyers of Iranian crude oil to slash their purchases.

U.S. and European sanctions have more than halved Iran's oil exports from about 2.2 million barrels per day before the measures took effect in 2011, costing the country billions of dollars a month in lost revenues.

Howard Berman, a Democrat and former chairman of the House of Representatives Foreign Relations Committee, suggested that sanctions should not be eased without Iranian action to rein in its nuclear program.

"Sanctions brought us to where we are and I think it would be a big mistake, and I think the administration knows it would be a big mistake, to get rid of or weaken those sanctions absent meaningful agreement with Iran," he said.

"If Iran were to make a decision to unilaterally suspend their enrichment program as a show of good faith I think it would allow things to move quickly from there," he added. "We should be trying to strive for a big deal, not nibbling around the edges."

Uranium enrichment can produce fuel for power plants or, if extended, fissile material for atomic bombs.