Iran accused the United States on Wednesday of involvement in the disappearance of one of its nuclear scientists during a pilgrimage to Saudi Arabia, raising a new mystery at a time when the West is trying to determine the extent of Iran's nuclear program. Shahram Amiri vanished during a pilgrimage to the kingdom more than four months ago and so far Saudi Arabia has not responded to requests for information on his whereabouts, Iranian officials say. But in complaints about his disappearance, Iranian officials have avoided even mentioning that Amiri was involved in nuclear research - a sign of the sensitivities surrounding the case. His disappearance came months before the revelation of a second uranium enrichment facility near the city of Qom that the United States accuses Iran of building secretly, a claim Teheran denies. The timing has raised speculation that Amiri may have given the West information on it or other parts of Iran's nuclear program. Iran's announcement of the disappearance also comes as it has entered landmark nuclear negotiations with the United States and other world powers, talks that have somewhat eased rising tensions between the two sides. President Mahmoud Ahmadinejad on Wednesday praised last weeks negotiations in Geneva, calling them "positive" and saying that have "led to a better dialogue." The United States and some of its allies accuse Iran of secretly seeking to develop nuclear weapons, a claim Iran denies, saying its program is intended only to produce electricity. Amiri worked as a researcher at Teheran's Malek Ashtar University, which is believed to be run by the elite Revolutionary Guard military corps. The university has been cited in the past by the UN for experiments connected with the nuclear program. Relatives cited in Iranian media said Amiri was researching medical uses of nuclear technology at the university and that he was not involved in the broader nuclear program. One Iranian news Web site, however, claimed Amiri had worked at the Qom facility and had defected in Saudi Arabia. The Web site, Jahannews, which is connected to Iranian conservatives, gave no source for the report. Amiri's wife and other relatives have demonstrated in recent weeks in front of the Saudi Embassy in Teheran, demanding to know his fate. His wife said he traveled to Saudi Arabia on May 31 for Omra, an Islamic pilgrimage, and that the last she heard from him was in a June 3 phone call, according to the semiofficial ISNA news agency. Foreign Minister Manouchehr Mottaki said Wednesday that Amiri had been arrested and accused the United States of a role. "We've obtained documents about US involvement over Shahram Amiri's disappearance," Mottaki said, according to the semiofficial Fars news agency. "We hold Saudi Arabia responsible for Shahram Amiri's situation and consider the US to be involved in his arrest," Mottaki said, quoted by the official IRNA news agency. Iran has asked Saudi Arabia for information on Amiri's whereabouts but has received no reply, Foreign Ministry spokesman Hasan Qashqavi said earlier this week. There was no immediate comment from Saudi officials. In Washington, State Department spokesman Ian C. Kelly said he had no information about the matter. "The case is not familiar to us," Kelly said. The Arab newspaper Asharq Al-Awsat, which is owned by Saudi businessmen, reported last week that Mottaki made a formal complaint to UN chief Ban Ki-moon about the disappearances of Amiri and several other Iranians in recent years, some of whom it feared may have provided nuclear information to the West. Qashqavi this week denied the complaint made any mention of the nuclear issue. Also on the list Mottaki handed over was Ali Reza Asghari, a retired general in the elite Revolutionary Guard and a former deputy defense minister, who disappeared during a private visit to Turkey in December 2007. Iran accused Western intelligence services at the time of possibly kidnapping the official, though other reports have said he may have defected. Another Iranian on the list was a man identified only by his last name, Ardebili, who was reportedly arrested in the Caucasus nation of Georgia recently. Qashqavi said Monday that Ardebili was a businessman and accused Georgian authorities of arresting him and handing him over to the United States. Asharq Al-Awsat identified him as a nuclear scientist, but gave not sourcing for the claim. A Georgian government spokesman in Tblisi refused to comment. Last month, Iran revealed that it was building the new enrichment facility outside Qom, bringing US and European accusations that it had been hiding the project. Teherandenied it sought to deceive the UN nuclear watchdog, saying it revealed the site earlier than required under its deals with the agency. The agency disagrees. After last week's talks in Geneva, Iran agreed to allow UN inspectors into the Qom facility on Oct. 25. It also is discussing a proposal to send some of its enriched uranium abroad for further enrichment to use in a research reactor in Teheran. The uranium would be enriched to a level of 20 percent for the reactor, up from the around 5 percent Iran has succeeded in reaching. Ahmadinejad on Wednesday said several nations - including the United States - had told Iran they were prepared to provide the further enriched uranium. "The United States has expressed readiness to provide 20 percent (enriched uranium) to Iran. We buy fuel from any country offering it. The US can be one of the sellers," IRNA quoted him as saying.