Iran and the U.N. nuclear watchdog have reached an agreement on a timetable to respond to lingering questions over Tehran's controversial nuclear activities, both sides said Tuesday.

The agreement was announced at the end of two days of talks in Tehran between senior officials from the International Atomic Energy Agency and Iranian nuclear negotiators.

"We have now in front of us an agreed working plan. We agreed on modalities on how to enforce it. We have a timeline for the implementation," IAEA chief of delegation Olli Heinonen told reporters.

Senior Iranian nuclear negotiator Javad Vaeedi also said the two sides agreed on a framework to resolve the outstanding issues over Tehran's nuclear activities. "The talks produced very great and constructive progress," he said.

Iranian and IAEA officials did not elaborate or provide more details on the timetable. But the agreement was expected to provide for easier inspection of Iran's nuclear facilities by the IAEA and urge Tehran to provide detailed answers on remaining questions over its nuclear activities.

The two sides did not say what the lingering questions were, but it is believed that they include questions over Iran's past experiments with small amounts of plutonium.

The talks in Tehran were seen as critical because they will be the basis for a progress report IAEA chief, Mohamed ElBaradei, is scheduled to deliver before the agency's September meeting.

Heinonen said he expected that report to be ready by next week and would include details from the Tehran talks.

Past IAEA reports have expressed concerns that Tehran has secretly developed elements of a more sophisticated enrichment program than the one it has made public; that it might not have accounted for all the plutonium it processed in past experiments and that its military might have been involved in enrichment, a program that Tehran insists is strictly civilian-run.

Two previous rounds of talks, in Vienna and Tehran earlier this summer, appeared to have improved relations between Iran and the U.N. agency.

But the talks coincided with a warning from Iran's top nuclear negotiator to the U.N. Security Council that imposing new sanctions against Iran will make Tehran's cooperation with the U.N. nuclear watchdog "fruitless," state television reported Tuesday.

Ali Larijani also accused the United States of seeking to undermine the progress achieved during talks between Iran and the IAEA in order to heighten tensions.

"If they take any illogical move (to impose new sanctions), the trend of Iran's cooperation with the International Atomic Energy Agency as well as the talks will become sterile," state television quoted Larijani as saying.

Larijani said the U.S. and its allies won't achieve anything by coercion and threats.

"What have they achieved from their hardline approach so far? Have they succeeded stopping our nuclear technology?" Larijani asked.

The United States and some of its allies fear Tehran is using its nuclear program as a cover to produce atomic weapons. Iran denies the charge, saying its program is solely geared toward generating electricity.

Iran has been agreeing to make new concessions in an apparent attempt to stave off a third round of sanctions. Tehran last month offered a rare concession and allowed IAEA inspectors to revisit a heavy-water reactor under construction outside Arak, central Iran, that has been off-limits since April.

The Arak facility could be used to produce plutonium once it is completed sometime in the next decade, and the Security Council has demanded Iran stop construction. Plutonium, like uranium, is a possible pathway to building nuclear arms.

The U.N. Security Council has already slapped two sets of sanctions, mostly economic and mainly targeting Iran's nuclear and missile programs. Iran has rejected the sanctions as illegal, saying it won't give up its right under the Nuclear Nonproliferation Treaty to enrich uranium and produce nuclear fuel.