The United States will press for new, tougher United Nations sanctions against Iran when diplomats hold a strategy session next week with five other nations that have sometimes reluctantly used the world body's punitive powers against oil-rich Iran.

The State Department said the six nations will meet Sept. 21 in Washington. Undersecretary of State Nicholas Burns, the Bush administration's lead negotiator on Iran sanctions, will represent the United States.

The U.N.'s five permanent members, all nuclear powers, plus Germany have tried for nearly two years to use sanctions or the threat of them to persuade Iran to drop disputed nuclear work. Two rounds of mild sanctions, meant more to shame Iran than to pinch it economically, have not slowed or stopped the disputed activities.

The United States, which has been estranged from Iran since the 1979 storming of the U.S. Embassy in Tehran, has been the prime mover for world sanctions. U.S. diplomats have not been specific about what they want the U.N. Security Council to try next. The six powers may consider a new round of sanctions in October, and some European diplomats are predicting contentious negotiations.

"We of course would have liked to have seen a new Security Council resolution drafted by now, but again this works on the timetable of the Security Council which isn't always our timetable," State Department spokesman Sean McCormack said Wednesday.

"We're confident we'll be able to move forward and get a new Security Resolution," McCormack added.

A senior Iranian envoy Wednesday accused Western nations of "poisoning the environment" at a key meeting of the U.N. nuclear watchdog agency in Vienna. Ali Ashgar Soltanieh, Iran's chief delegate to the International Atomic Energy Agency, said the United States and its allies are unfairly downplaying initial successes of an agency probe into his nation's nuclear past.

But the U.S. and its western allies suspect Iran is using limited cooperation with the IAEA as a smoke screen to deflect attention from its continued defiance of the Security Council.

A statement by the European Union Tuesday appeared to reflect that concern; it focused mostly on Iranian and said little about IAEA chief Mohamed ElBaradei's reported successes in prying answers from Iran.

Despite their reservations about the plan, the most vocal backers of new U.N. sanctions have toned down initial criticism of the IAEA probe or Iranian compliance.

Too much criticism could open the U.S., France and the U.K to the charge that they don't care about resolving the issue that had originally sent Iran's nuclear file to the Security Council -- Tehran's refusal to cooperate in dispelling suspicions about past nuclear activities.