A senior U.N. nuclear agency official said cooperation with Iran was "good" ahead of talks on Monday about Tehran's disputed atomic work, after an Iranian warning that new U.S. sanctions could harm ties with the agency.

Iranian news agencies quoted Olli Heinonen, deputy director of the U.N. International Atomic Energy Agency (IAEA), as making the brief comment upon arrival in Tehran for a new round of negotiations with senior Iranian officials.

Iran and the Vienna-based nuclear watchdog agreed in August on a timetable to answer outstanding IAEA questions about the country's atomic activities, prompting world powers to postpone a third round of U.N. sanctions until at least November.

Monday's meeting takes place amid rising Iran-U.S. tension. Washington, which has said the IAEA-Iran deal fails to address the core U.N. demand that the country halt sensitive nuclear work, last week imposed new bilateral sanctions on Iran.

Heinonen entered into talks with Iran's deputy nuclear negotiator, Javad Vaeedi, the official IRNA news agency said, adding the discussions may continue until Wednesday. Previous rounds since the August deal have also lasted several days.

Iran rejects accusations it is seeking atom bombs and former chief nuclear negotiator Ali Larijani, who remains an influential figure, on Friday said the latest U.S. measures could push Tehran to rethink its relations with the IAEA.

But when asked about Iran's cooperation with his agency, Heinonen said it was "good" even though much remained to be done, according to IRNA and Iran's Mehr news agency.

"We have done many things, but much work remains and I hope we can do that," IRNA quoted him as saying at Tehran's airport.

Iran, which stonewalled IAEA investigators for years, is to provide answers in phases by the end of the year under the August agreement.

Mohammad Saeedi of Iran's Atomic Energy Organisation voiced hope negotiators in this round could wrap up talks on Iranian centrifuges used to enrich uranium, which Tehran says is for fuelling power plants but the West fears has military purposes.

"We had two rounds of extensive talks with the agency, with clear and frank discussions," Mehr quoted him as saying. "Iran will continue cooperation as long as the agency has questions."

Iran uses a 1970s vintage of centrifuge, called P-1s, prone to breakdown if spun at high speed for long periods. It is researching an advanced P-2 model, which can refine uranium much faster, at sites off limits to IAEA inspectors.

IAEA Director Mohamed ElBaradei will report to the IAEA's 35-nation board of governors in mid-November. If Iran has not answered sensitive questions by then, Western powers say they will move to have harsh U.N. sanctions adopted against Iran.

The U.N. Security Council has imposed two sets of limited sanctions on Iran for its refusal to halt enrichment.