Iran has not invited the United States to visit its nuclear sites, diplomatic sources close to the UN's nuclear agency IAEA said Tuesday, after Tehran announced it would open its sites.

Among the six world powers involved in talks over Tehran's atomic programme, only Russia and China had been invited, sources in Vienna said.

The United States, Britain, France and Germany, on the other hand, had not received an invitation, they added.

In a letter dated December 27, Iran invited ambassadors to visit its nuclear sites of Bushehr and Natanz on January 15 and 16, according to diplomatic sources Tuesday.

China, a close ally and economic partner of Iran, already confirmed that it was among those invited to visit the atomic sites.

Among the other invitees were Hungary , as rotating president of the European Union, Egypt, Cuba and representatives of the Arab League and the Non-Aligned Movement, diplomatic sources in Vienna said.

In a rare move, Iran said Tuesday it had sent invitations to ambassadors of some of the countries represented in the International Atomic Energy Agency (IAEA) to visit its nuclear sites.

"The representatives of some European Union countries, NAM (Non-Aligned Movement), and some representatives of the five-plus-one (six world powers) have been invited to visit our nuclear sites," Iranian foreign ministry spokesman Ramin Mehmanparast told reporters.

He said the invitation was part of the Islamic republic's attempt to demonstrate "cooperation with the IAEA."

The IAEA only said it had not seen the invitations and could not comment on them.