The United Nations nuclear watchdog chief said Tuesday that he had reached an agreement with Iran on probing suspected work on atomic weapons, adding that he hoped the agreement would "be signed quite soon."

Yukiya Amano, head of the International Atomic Energy Agency (IAEA), spoke a day after holding rare talks in Tehran and a day before Iran and six world powers will hold broader negotiations on the extent of Tehran's nuclear program.

"(A) decision was made to conclude and sign the agreement... I can say it will be signed quite soon," Amano told reporters at Vienna airport after returning from Tehran.

Amano, who had been looking for a deal giving his inspectors a freer hand to investigate suspected atomic bomb research in Iran, described the outcome of his meetings in Iran as an "important development".

He said "some differences" remained but that Iranian chief nuclear negotiator Saeed Jalili had told him these would not pose an obstacle to an agreement.

Jalili will sit down in Baghdad on Wednesday with senior officials from the six world powers involved in efforts to resolve the Iranian nuclear stand-off peacefully.

"We understand each other's position better," Amano said about his talks with Jalili and other Iranian officials.

He said he had raised the issue of access to the Parchin military site - an IAEA priority in its inquiry - and that this would be addressed as part of the agreement's implementation.

The IAEA investigation has been stalled for more than four years, with Iran saying it has never carried out such experiments. Iran denies that it is interested in nuclear weapons, saying it wants nuclear power only to generate energy.

On Monday, the U.S. Senate unanimously approved on a package of new economic sanctions on Iran's oil sector, just days ahead of a meeting in Baghdad between major world powers and Tehran.

The new sanctions build on penalties signed into law by U.S. President Barack Obama in December against foreign institutions trading with Iran's central bank. Those sanctions already have cut deeply into Iran's oil trade.

The new package would extend sanctions to cover dealings with the National Iranian Oil Co and National Iranian Tanker Co, aiming to close a potential loophole that could have allowed Tehran, the world's third-largest petroleum exporter, to continue selling some of its oil.