Foreign ministers from the Group of Eight (G8) major industrial powers ratcheted up pressure on Iran Thursday over an international nuclear offer, giving Tehran a week to come up with a "clear and substantive" reply.

The five permanent members of the UN Security Council (P5) -- Britain, China, France, Russia and the United States -- plus Germany offered in mid-June a package of incentives aimed at persuading Iran to halt nuclear enrichment activities, which lie at the heart of Western fears that Tehran could develop nuclear weapons.

Iran has repeatedly said its nuclear program is only for peaceful purpose.

"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.

Speaking at a press conference after the meeting, Russian Foreign Minister Sergei Lavrov said: "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."

The call on Iran to respond quickly to the offer came as G8 ministers met two weeks before Russian President Vladimir Putin hosts other G8 leaders for a summit in his hometown of St. Petersburg.

The new package for Iran contains economic and political incentives, including talks with the United States, Western help to build nuclear reactors for Iran, a guaranteed supply of nuclear fuel and permission for Iran to buy aircraft and spare parts if Tehran suspends uranium enrichment.

Iranian President Mahmoud Ahmadinejad has said his country would formally reply to the six-nation proposals by mid-August.

U.S. Secretary of State Condoleezza Rice described the offer as being "very favorable" and said G8 ministers hope for a "serious response" from Iran.

"The P5+1 (Germany) will assess the situation when we have heard from the Iranians after Javier's meeting," Rice said.

But Tehran brushed aside calls for a speedy response on Thursday.

Iranian Foreign Minister Manouchehr Mottaki said at the UN headquarters in New York that Iran is seriously and carefully reviewing the proposed package, stressing that "a response will be in August."

Iran's response will be clear and substantive, he said, adding that the proposed package contains questions and ambiguities which must be cleared.

Mottaki said Solana may be able to answer some of Tehran's questions in the talks on July 5.

However, "questions and ambiguities on the Iranian side are pending. Therefore, we welcome the discussions and negotiations for clarification of those ambiguities," he said.

The G8 diplomats meeting in Moscow said they expected the foreign ministers of P5+1 and Solana to provide their assessment by mid-July.