VIENNA » Diplomats say Syria has reneged on a promise to quickly cooperate with a U.N. probe of its secret nuclear activities.

Damascus promised such cooperation in June. But the diplomats say it now has announced it cannot provide more information to challenge a U.N. assessment that it tried to build a plutonium producing reactor until late fall.

The diplomats asked for anonymity because their information is confidential. They said Thursday that U.N. nuclear chief Yukiya Amano will announce next month that he has been stymied in his Syria probe.

The International Atomic Energy Agency referred Syria to the U.N. Security Council in June for stonewalling IAEA investigative efforts. The U.N. nuclear agency says Syria tried to build a reactor meant to produce plutonium, which can be used to arm warheads.