The United Nations nuclear agency's chief inspector says there is “still a lot of work to be done” with Iran after three days of “intensive” talks about its atomic energy program.

Herman Nackaerts said Wednesday both sides claim commitment to resolving “outstanding issues” and that inspectors have planned another trip for the near future.

A team of six senior International Atomic Energy Agency inspectors met with Iranian officials about suspicions the country's uranium enrichment program is designed to produce nuclear weapons. Iran says the program is solely for generating electricity.

Iran on Tuesday described the talks as “constructive.” Iranian media reported the inspectors did not visit any nuclear sites, saying only “technical and legal issues were discussed.”

In recent weeks, Iran has said frequently in recent weeks that it is prepared to resume talks with world powers — talks which were suspended more than a year ago.

Meanwhile, U.S. lawmakers have proposed tighter economic sanctions on Iran, as senior intelligence officials say measures already in place are having an effect.

Lawmakers in Washington unveiled proposals Tuesday for sanctions that would target Iran's national oil and shipping companies and restrict its ability to tap into electronic banking services.

Tensions have risen in recent weeks since the United States and and the European Union imposed the toughest sanctions yet, in a drive to force Iran to provide more information about its nuclear program. The measures aim to block crude oil sales by Iran, OPEC's second largest exporter.

U.S. Central Intelligence Agency Director David Petraeus said Tuesday that sanctions are hurting Iran now more than ever before.

Stephen Zunes, a Mideast analyst at the University of San Francisco, told VOA the sanctions are having an economic impact, but that the political impact remains to be seen.

“If they end up hurting ordinary Iranians more than the Iranian elites, it may get people to rally around the flag and may give the regime an excuse to confront the growing popular upset at their misguided economic policies domestically. So, as in any kind of sanctions, a fine line between putting pressure on the regime and creating an atmosphere where it will create the kind of popular backlash that could lead to compromise.”

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