* Amiri arrives in Tehran, thanks authorities
* Washington says Amiri was in U.S. of his own free will (adds quotes from news conference, background)
By Parisa Hafezi and Ramin Mostafavi
TEHRAN, July 15 (Reuters) - A nuclear scientist who says he was abducted by U.S. agents thanked Iranian authorities for returning him home on Thursday, the culmination of a murky saga that has underscored deep U.S.-Iranian mistrust.
Washington denied kidnapping Shahram Amiri and insisted he had lived freely in the United States. A U.S. official said, however, that the United States, eager for details of Tehran's nuclear programme, had obtained "useful" information from him.
Wearing a beige suit, a smiling Amiri made victory signs as he hugged his tearful son and wife, who greeted him at Tehran's International Imam Khomeini Airport, along with other family members and a senior foreign ministry official, Hassan Qashqavi.
"Americans wanted me to say that I defected to America of my own will to use me for revealing some false information about Iran's nuclear work," Amiri told a short news briefing at the airport.
Qashqavi thanked Amiri for his "resistance to pressure".
Iran accuses the CIA of kidnapping Amiri, who worked for Iran's Atomic Energy Organisation, a year ago while on a pilgrimage in Saudi Arabia. He surfaced at the Iranian interests section of the Pakistani embassy in Washington on Monday.
Iran is locked in a dispute with the United States and its allies over Tehran's nuclear programme, which the West says is designed to produce nuclear weapons and Iranian officials say aims to generate power.
The mystery surrounding Amiri fuelled speculation that he may have passed information about Iran's nuclear programme to U.S. intelligence. ABC News reported in March that Amiri had defected and was helping the CIA.
The State Department said the United States did not kidnap Amiri, but it has not addressed whether another country might have abducted him and turned him over.
Amiri had painted a dramatic picture of a cloak-and-dagger operation to abduct him.
"While I was on the pilgrimage in Saudi Arabia, a car offered me a lift...a gun was pointed at me as soon as I got in the car," Amiri had told state television. "Then I was drugged ... I was transferred to America by a military plane."
U.S. State Department spokesman P.J. Crowley appeared to anticipate further accusations from Tehran on Amiri's return.
"Once he gets back to Iran I suspect that he'll have a variety of things to say and my advice would be to take what he says with a grain of salt," he said.
A man identifying himself as Amiri has variously said in recent videos that he was kidnapped and tortured, that he was studying in the United States and that he had fled U.S. agents and wanted human rights groups to help him return to Iran.
Intelligence about the Iranian nuclear programme is at a premium for the United States, which fears that a nuclear-armed Iran would threaten its close ally, Israel, as well as oil supplies from the Gulf, and friendly nations in Europe.
Asked why Amiri was going back, a U.S. official, who spoke on condition of anonymity, said the Iranian authorities could have put pressure on his family back home. (Editing by Elizabeth Fullerton and Alex Richardson)