Syria has reneged on a promise to quickly cooperate with a U.N. probe of its nuclear activities, saying it won't be able to provide more information to challenge an assessment that it tried to build a plutonium-producing reactor until October, diplomats told The Associated Press on Thursday.

The delayed cooperation will likely add to concerns that Damascus was in the early stages of a secret program that could be harnessed to produce nuclear weapons. It also comes as Syrian leader Bashar Assad faces unprecedented international pressure over his months-long crackdown on anti-government protesters, possibly deflecting attention in Damascus from dealing with the nuclear issue.

The two diplomats, who asked for anonymity because their information is confidential, said U.N. nuclear chief Yukiya Amano will tell board member nations of the International Atomic Energy Agency that he has been unable to make progress in his Syria probe at their 35-nation meeting starting Sept 12.

The board referred Syria to the U.N. Security Council in June for stonewalling IAEA attempts to investigate covert nuclear activities revealed in 2008, a year after Israeli warplanes bombed a secret Syrian desert site.

After trying in vain for three years to secure Syrian cooperation in its investigation, the IAEA drew on evidence available in June to assess that the target was in fact a nearly finished reactor, built with North Korean help, meant to produce plutonium.

Along with highly enriched uranium, plutonium can be used to arm nuclear warheads.

Syria has denied any secret nuclear activities. But the Arab country also has rejected IAEA requests for follow-ups to an initial visit to the desert site and access to others believed linked to it.

In a letter shared with the AP just before the Security Council referral in June, the Syrian government promised "full cooperation" with the probe. Since then, however, Amano has told the AP that meetings with Syrian nuclear officials have produced no results.

The diplomats said Thursday that the Syrians told the agency in a recent letter that they cannot meet requests for access to suspect sites and other information until October. Damascus did not provide substantial details about the reason for the delay.

Amano last month told the AP that _ unless the Syrians come forward with new information _ the agency stands by its assessment that the facility destroyed by Israeli jets in 2007 was meant to be a plutonium-producing reactor once completed.

The agency has produced regular written reports on its attempts to probe Syria's undeclared nuclear programs since 2008, but the diplomats said that won't be the case for the September board meeting, in another reflection of the fact that Damascus has provided no information to help the IAEA probe.

The U.N. Security Council met in closed session on July 14 to discuss the IAEA assessment. Some Western ambassadors said afterward that the agency's findings raised concerns that Syria violated its nonproliferation obligations.