Foreign ministers of the Group of Eight (G8) nations gather for talks in Moscow, June 29, 2006. -Xinhua/AFP

Foreign ministers from the Group of Eight (G8) major industrial powers said on Thursday they wanted a "clear and substantive" response from Iran next week to the international nuclear offer.

"We are disappointed in the absence of an official Iranian response to the positive proposal," the ministers said in a statement issued at the end of their meeting in Moscow.

"We expect to hear a clear and substantive Iranian response" at the planned meeting on July 5 of EU foreign policy chief Javier Solana and Iran's top negotiator Ali Larijani, the ministers added.

Tehran is studying an international package of incentives aimed at persuading it to halt nuclear enrichment, and Iranian President Mahmoud Ahmadinejad has said his country would formally reply to the six-nation proposals by mid-August.

The offer, details of which have not been made public, was backed by the five permanent members of the UN Security Council and Germany and presented to Iran by European Union foreign policy chief Javier Solana.

The call on Iran to respond quickly to the offer came as G8 ministers met for the last time before a summit of G8 leaders in St. Petersburg in two weeks' time.

Speaking at a press conference after the meeting, Russian Foreign Minister Sergei Lavrov described the offer as positive and being made in good faith.

The United States has accused Iran of secretly developing nuclear weapons under a civilian front, but Iran has repeatedly said its nuclear program is for peaceful purpose.

"We did not discuss anything beyond the offer, which we all made in good faith to Iran, which is a positive offer, and we expect a positive response," Lavrov said.

US Secretary of State Condoleezza Rice said G8 ministers expect a "serious response" from Iran and will assess "where we are" after Solana meets Larijani on July 5.

"We expect the foreign ministers of Britain, Germany, China, Russia, the United States and France, and also the European Union's high representative for common foreign and security policy to come up with their assessment by mid-July," the G8 ministers said in the statement.