US Secretary of State Condoleezza Rice says that a mid-July deadline is still in place for Iran to accept an offer of incentives aimed at curbing its nuclear ambitions. She warned the government in Tehran against stalling.
"After receiving some news from Brussels that there are assassins' squads... security officals voiced concern about the safety of the Iranian delegation" led by negotiator Ali Larijani, the state run news agency IRNA said.
"Therefore today's trip of Mr Larijani and the accompanying delegation to Brussels was cancelled," it added.
"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.
However IRNA said that Mr Larijani had "in order to show goodwill accepted a dinner invitation" for tomorrow, in line with a demand from the EU's top diplomat Javier Solana.
Earlier, Mr Solana also rejected Iranian attempts to buy time to respond to the international offer on its nuclear program, giving Mr Larijani one more day to meet for talks that were due to have taken place on Thursday.
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 Iran's ISNA news agency.
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 Mujahideen.
The group is also outlawed in the EU and the United States. MrRajavi is based in France and is one of the fiercest critics of Iran's clerical regime.
He 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 is awaiting Iran's answer to an offer of economic and political incentives in exchange for a suspension of uranium enrichment. Tehran could face sanctions if it rejects the proposal.