The diplomats said no date for the inspection had been set. But one of them suggested it could come before the planned Feb. 28 meeting of the board of the International Atomic Energy Agency - the U.N. watchdog group now investigating Tehran's nuclear activities.

One of the diplomats said the Iranians gave their ''agreement in principle'' about 10 days ago.

IAEA inspectors last visited the Parchin complex just outside Tehran in mid-January in their search for radiation that could point to such research. Diplomats accredited to the agency or familiar with its work said the inspectors were given partial access to the site and were allowed to take environmental samples for analysis.

Washington has accused Iran of being part of an ''axis of evil'' with North Korea and prewar Iraq. The United States alleges Iran may be testing high-explosive components for nuclear weapons.

Diplomats last month said that as far as they knew the IAEA experts were not impeded in their first visit, beyond the limitations placed on where they could take their samples.

In leaks to the media last year, U.S. intelligence officials said a specially secured site at Parchin may be used in research for high-explosive components of nuclear weapons.

Iran insists its military is not involved in nuclear activities, and the IAEA has found no firm evidence to the contrary. The agency also has not been able to support U.S. assertions that nearly two decades of covert nuclear programs discovered more than two years ago were aimed at making nuclear weapons - not generating electricity, as Tehran claims.