TEHRAN (Reuters) - Iran said on Tuesday it had invited some envoys accredited to the UN nuclear watchdog to visit important nuclear facilities this month, shortly before a second round of talks between Tehran and major powers. Those invited included representatives from some of the six world powers involved in diplomatic efforts to resolve the dispute over Iran's nuclear programme, Foreign Ministry spokesman Ramin Mehmanparast said without giving details. Iran's envoy to the International Atomic Energy Agency (IAEA), the Vienna-based UN nuclear body, said the plan was for the ambassadors to travel to the Natanz uranium enrichment facility and the Arak heavy water reactor. The two sites are at the heart of Iran's nuclear dispute with the West, which suspects the Islamic Republic is seeking to develop nuclear weapons. Iran denies this. "Ambassadors ... are invited to visit our nuclear sites, particularly in Natanz and Arak," Soltanieh told Reuters in Vienna. ""This is in the line of our transparent nuclear policy," he said, adding that meetings with high-ranking Iranian officials would also be organised. Analysts said the move was an attempt by Tehran to demonstrate openness about its nuclear programme before meeting the six powers - Russia, China, the United States, France, Germany and Britain, known as P5+1 - in Istanbul this month. "This is Iran trying to show it is flexible, that we don't have any problem to show our facilities to diplomats," said Mahjoob Zweiri, an Iran expert at the University of Qatar. "It will be a clear message to P5+1 that Iran will not retreat in the negotiations," he said. The IAEA regularly visits Iranian nuclear sites including Natanz, but it has voiced growing frustration at what it sees as lack of Iranian full cooperation with its inspectors. Mehmanparast said the invited ambassadors also included some European Union states as well as envoys from the non-aligned movement of mainly developing countries. The invitation "has once again shown the goodwill of our country regarding cooperation" with the IAEA and Iran's "peaceful nuclear activities," Mehmanparast said. "They will possibly visit Iran and our nuclear facilities on January 15 and 16," he told a news conference. Meanwhile, the Pentagon denied on Monday that Iran had shot down any of its drone surveillance aircraft in the Gulf but acknowledged some spy planes had crashed in the past due to mechanical failure. Irans semi-official Fars news agency quoted a senior Revolutionary Guards commander on Sunday as saying Iran had shot down two unmanned Western reconnaissance drone aircraft in the Gulf. Commander Amir Ali Hajizadeh, head of the elite forces aerospace unit, did not say when the alleged incidents took place or produce proof that they had happened. Pentagon spokesman Colonel Dave Lapan said there were no recent reports that would corroborate what the Revolutionary Guard said about unmanned aerial vehicles. Another US military official, speaking on condition of anonymity, told Reuters the last incident he was aware of involved a drone aircraft crashing in the Gulf in early 2009. Like others before it, the crash was due to mechanical failure and the aircraft landed in international waters, the official said. We have had cases in which UAVs have gone down in the Gulf ... but I dont have any indication (of a case) where a UAV has been taken down by hostile fire in the Gulf, the official said. After a US military review of past crashes, the official said there were no reports of any UAVs being shot down over the Gulf at any time in the past. Iran is at odds with major powers over its nuclear activities, which the US and its allies suspect are aimed at producing a nuclear weapon. Tehran has denied the allegations and said it wants only to generate electricity. The United States and Israel, Irans main foes, have not ruled out military action if diplomacy failed to end the nuclear row. Iran has dismissed reports of possible US or Israeli plans for a military strike, but said it would respond by attacking US interests and Israel in the event of such an assault. Analysts say Tehran could retaliate by launching hit-and-run strikes in the Gulf and by closing the Strait of Hormuz. About 40 percent of traded oil leaves the Gulf region through the strategic waterway. Meanwhile, an Iranian court Tuesday lifted a ban on a leading reformist weekly, a semi-official news agency said, while keeping at least a dozen more off the streets in a crackdown dating to the disputed 2009 presidential election. The Chelcheragh weekly had been closed in November by the Press Supervisory Board for publishing articles the Board said offended public morals. Prior to its closure, Chelcheragh had received two warning from the authorities. After Chelcheraghs managing director gave some explanations to the board about one of its articles ... judiciary officials lifted the ban, Mehr news agency reported, adding the weekly would return to newsstands Saturday. Published on Saturdays, the weekly has been popular among the young for its articles on culture, art, cartoons, satire and sports.