The call for further sanctions followed a one-page letter sent by Iran to major powers on Tuesday containing no reply to an offer by Russia, China, the US, Germany, Britain and France of further talks and an incentives package if Iran freezes expansion of its nuclear work.

Western powers accuse Iran of attempting to develop nuclear weapons but Iran says its programme is only for peaceful energy purposes.

'No firm agreement'

However, Vitaly Churkin, Russia's ambassador to the UN, said there was no definite agreement among the six powers for a fourth round of sanctions against Iran.

"It may well be that in the course of those discussions some members of the six raised the issue of the sanctions," Churkin said.

"But to the best of my knowledge there has been no firm agreement or understanding or concerted work in this regard."

Churkin also said Moscow had not set a deadline for Iran to respond to the offer, and that ministers from the six powers would meet in September to discuss Iran.

Al Jazeera's correspondent at the UN, John Terret, said according to UN sources, nothing concrete would happen until the UN General Assembly meets in September, when the six powers would have the chance to try to convince Iran to get on board or get the unanimity needed for sanctions.

Also, a senior official from the International Atomic Energy Agency, the UN nuclear watchdog, is heading to Tehran on Thursday to try to offer the Iranians the clarification they are seeking on the offer, he said.

'Clear choice'

Kim Howells, a British foreign office minister, said the UK was disappointed by Iran's response so far.

"Iran has a clear choice: Engagement or isolation. We regret that Iran's leaders appear to have chosen isolation," he said.

"If Iran continues to refuse to come to the negotiating table, the international pressure on Iran will only grow," he said.

The UK had warned that a lack of a positive answer from Tehran by the end of Tuesday would prompt the six powers to ask the UN Security Council to take further punitive measures.

The new deadline was set after Iran ignored a previous demand to respond by last weekend to the proposed package, insisting no such deadline had been set.

The UN has already imposed three sets of sanctions against Iran over the dispute.

Israel, which is concerned about a threat from a potentially nuclear Iran, said on Thursday that Iran was trying to buy time by giving "evasive" replies and the UN should impose strong sanctions.

"Every day that the centrifuges continue to spin brings Iran closer to a nuclear weapons capability and threatens the stability and security of the region and the world," Sallai Meridor, Israel's ambassador to the United States, said.

Tensions were again heightened on Monday when Iran said it had successfully test-fired an anti-ship missile with a range of 300km that would allow it to close the Strait of Hormuz between Iran and Oman, one of the world's busiest oil shipping routes.