The US government shutdown has hampered Washington's ability to enforce punitive measures on Iran's nuclear program, he head US negotiator with Iran said Thursday.

The lack of funding and government workers has curtailed American abilities to enforce substantive pressure on Tehran, said Wendy Sherman, under secretary of state for political affairs.

“Our ability to do that, to enforce sanctions, to stop sanction evaders, is being hampered significantly by the shutdown,” AFP cited Sherman as saying to the US Senate Foreign Relations Committee.

The US government began a partial shutdown on Tuesday for the first time in 17 years, potentially putting up to 1 million workers on unpaid leave.

Sherman has vowed that the United States was looking for Iran to take specific steps to slow its uranium enrichment and to open a wider window into its nuclear program  ahead of October 15-16 negotiations with Tehran.

Sherman urged US lawmakers to hold off on imposing additional sanctions against Iran ahead of the talks.

In testimony for Congress, Sherman held out the possibility of sanctions relief for Iran, but she made it clear the United States expected concrete actions from Tehran before this could happen and said all US 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. "We have been clear that only concrete viable steps, and verifiable steps, can offer a path to sanctions relief."

The United States, which broke diplomatic ties with Iran in 1980 after the Islamic revolution, and its allies suspect Iran is using its civilian nuclear program as a cover to develop nuclear weapons. Iran denies this, saying its program is for solely civilian and peaceful purposes.

Six major powers - Britain, China, France, Germany, Russia and the United States - will meet Iran's new negotiating team in Geneva on Oct. 15-16 to discuss its nuclear program.

The United States and its allies have imposed extensive sanctions against Iran, including a US law that forced buyers of Iranian crude to sharply cut their purchases, because of Tehran's failure to address their concerns about the nuclear program.

At the hearing, Senate Foreign Relations Committee Chairman Robert Menendez, a Democrat, said some US lawmakers are moving forward on new US sanctions to further cut Iranian petroleum sales, but held out the possibility of sanctions relief if Iran lives up to its UN Security Council obligations.

Those obligations include ceasing the enrichment of uranium, a process that can produce fuel for nuclear power plants or, if extended, fissile material for an atomic bomb.

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