At talks with Iran in Geneva on July 19, the six nations - China, Russia, the U.S., France, Britain and Germany - put forward a package of trade and nuclear technology incentives, demanding a response from Tehran within two weeks.
European Union foreign policy chief Javier Solana received Tuesday a letter from Tehran, which did not contain a clear response to the proposals.
Russian Foreign Ministry spokesman Andrei Nesterenko described Iran's lack of a definitive answer as unfortunate, but said the six world powers would continue their diplomacy.
"The Iran Six remains open to any possible actions in the framework of our coordinated dual-track strategy," he said. "We will pursue all possibilities to continue contacts with the Iranian side to clarify its position and find a diplomatic solution."
The Iran Six diplomats held telephone talks earlier Wednesday to agree how to respond to Tehran's unclear response to their offer to end the dispute.
"We haven't set any deadlines ourselves for their response and there is ongoing dialogue," Russian Ambassador to the UN Vitaly Churkin told a news conference in New York.
He said the Iran Six would hold a ministerial meeting on the Iranian nuclear program in September.
Meanwhile, the United States and Britain said the six had agreed to consider a new resolution at the U.N. Security Council on a fourth set of sanctions against Iran.
"While informal contact between Javier Solana and [Iran's chief nuclear negotiator] Saeed Jalili will continue, we have no choice but to pursue further sanctions against Iran," a spokesman for Britain's Foreign Office said.
Iran is currently under three sets of relatively mild UN Security Council sanctions for defying demands to halt uranium enrichment, which it says it needs purely for electricity generation.
The U.S. and Israel have refused to rule out military action against Iran, the world's fourth largest oil producer, if diplomacy fails to end the dispute.
