TEHRAN (Reuters) - A group of ambassadors to the UN atomic watchdog toured an Iranian nuclear site on Saturday, state television reported, and Tehran accused the European Union of missing an historic opportunity by boycotting the visit. Iran said the tour, which China and Russia also snubbed after being discouraged by Western officials, aimed to demonstrate the countrys transparency about its atomic programme before talks with major powers. Tehran invited some ambassadors accredited to the International Atomic Energy Agency to visit its nuclear sites. The U.S, British, French and German envoys were not asked along, while the EU declined its invitation, saying it was the task of UN nuclear inspectors to carry out such visits. The EU lost the historic opportunity for further cooperation with Iran and also visiting its peaceful nuclear activities, Ali Asghar Soltanieh, Irans IAEA envoy, told state TV before the tour. Analysts said the selective invitation may be aimed at eroding new harmony among the US and European governments on the one hand, and Russia and China on the other in next weeks talks with Iran, aimed at curbing its nuclear work. The United States and its allies accuse Iran of secretly working to make atomic bombs under cover of a civilian nuclear programme to generate electricity. Alongside the Non-Aligned Movement of developing nations, the group of seven envoys comprised ambassadors from Egypt, Venezuela, Syria, Algeria, Oman and Cuba. The envoys, who stay in Iran until Monday, toured the heavy water Arak installation. Later they will also visit the underground Natanz uranium enrichment site where feedstock uranium hexafluoride (UF6) gas is fed into centrifuges. This is to make material to fuel power plants which could, if greatly enriched, be used for nuclear warheads. Journalists working for foreign media in Iran were not invited to tour the sites. Washington and Israel, Irans arch foes, have not ruled out military action if diplomacy fails to end the standoff. Iran has warned P5+1 that the talks could be the last chance for the West because Tehrans atomic capability was improving. I hope ... P5+1 countries will use this opportunity to find a face-saving manner to settle this issue they have created, Irans top nuclear official and acting Foreign Minister Ali Akbar Salehi said at Arak. It will be a win-win situation for everyone. Some diplomats and analysts also said the tour could be a stalling tactic as Iran continued to build its stockpiles of enriched uranium in the teeth of international sanctions. However, US Secretary of State Hillary Clinton said on Monday that sanctions had set back Irans nuclear programme and given more time to persuade Tehran to change tack. The U.N Security Council, US and the European Union have been raising pressure on Tehran to suspend its uranium enrichment-related work by imposing tougher sanctions. Meanwhile, the semi-official Mehr news agency reported on Saturday that Iran will ban flights by all its Tupolev aircraft from February following a series of disasters involving Russian-built jets in the Islamic state. Iranian airlines have been dogged by accidents in recent years, prompting calls for a revamp of the countrys aging air fleet which also includes many US-built Boeings acquired before the 1979 Islamic revolution. Reza Nakhjavani, head of the Civil Aviation Organization, ordered four Iranian airlines to stop flying Tupolev-154s from February 20 due to past crashes and the expiry of airworthiness documents. Using Tupolev-154 planes is forbidden because of the recent air incidents and the expiry of the service date of this type of aircraft on February 19, Nakhjavani said in a letter. Altogether 17 Tupolev-154s, owned by Iran Air Tour, Kish Air Eram and Taban airlines, are currently in service in Iran.