Inspectors from the United Nations nuclear watchdog have completed a three-day visit to Iran with no apparent breakthrough on Tehran's controversial atomic energy program after Western powers imposed tough, new economic sanctions.

The semi-official Fars news agency Tuesday called the talks between Iranian officials and the visiting team “constructive,” saying the two sides had agreed to continue their dialogue. The Fars report said the date of future talks had been set, but did not give details.

The senior International Atomic Energy Agency inspectors traveled to Tehran Saturday for meetings with Iranian officials about suspicions that the Islamic state's uranium enrichment program is designed to produce nuclear weapons. Iran says the effort is solely for generating electricity.

Iranian media reported the six-person IAEA team did not visit any nuclear sites, saying only “technical and legal issues were discussed.” Also Tuesday, the semi-official ISNA news agency said some hardline Iranian students had gathered in front of the country's Atomic Energy Organization to protest the inspectors' visit.

Tensions with the West rose when the U.S. and European Union recently imposed the toughest sanctions yet in a drive to force Tehran to provide more information on its nuclear program. The measures aim to block crude oil sales by Iran, OPEC's second largest exporter.

In Washington, U.S. lawmakers unveiled proposals for even tighter sanctions that would single out Iran's national oil and shipping companies and restrict its ability to tap into electronic banking services.

Iranian Foreign Minister Ali Akbar Salehi Monday offered to extend the IAEA visit. Iran has also said frequently in recent weeks that it is prepared to resume talks with world powers, which were suspended more than a year ago.

But hardline Iranian leaders have also threatened to close the vital Strait of Hormuz shipping lanes if sanctions prevent the country from exporting crude. Washington has said it would not tolerate a blocking of the waterway.