"After receiving some news from Brussels that there are assassins squads ..., security officals voiced concern about the safety of the Iranian delegation," it said.

"Therefore today's trip of Mr Larijani and the accompagnying delegation to Brussels was cancelled," it said.

"After the European side gave the necessary guarantees to secure the lives of the Iranian delegation, it was decided that the session would be held next Tuesday," said IRNA, quoting an informed source.

It added, however, that Larijani had "in order to show goodwill accepted a dinner invitation" for Thursday.

Earlier, the EU's top diplomat rejected Iranian attempts to buy time to respond to an offer aimed at curbing its nuclear ambitions, giving Tehran's negotiator one more day to meet for talks due to have taken place on Wednesday.

Officially, Iran decided to delay the Brussels meeting due to the possible presence in the Belgian capital of a loathed exiled opposition leader, according to an Iranian media report.

"The Europeans had been told to prevent the presence of the hypocrites' chief in Brussels in this period, but they were late to act. This was the reason for the cancellation of the trip," the ISNA news agency said.

The reference was to Maryam Rajavi, leader of the National Council of Resistance of Iran, a banned opposition group connected to the Iraq-based armed People's Mujahedeen.

The group is also outlawed in the EU and the United States.

Rajavi, based in France and one of the fiercest critics of Iran's clerical regime, cancelled a planned address to the European Parliament, saying that she did not want it serve as a "pretext" to halt the nuclear talks.

The UN Security Council members is awaiting Iran's answer to an offer of economic and political incentives in exchange for a suspension of uranium enrichment.

"I had made clear to the Iranians and to Dr Larijani that we want to proceed rapidly to examine together the ideas I put to him early last month," Solana said, after the Iranians delayed the talks at the last minute.

The EU diplomat's spokeswoman, Cristina Gallach, said that Solana told Larijani that "waiting another week was impossible" and that "there had to be a contact before that."

A further round of talks has already been pencilled in for next Tuesday, the spokeswoman said.

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