The semi-official Fars news agency reported earlier in the day that border guards had detained seven US troops as they tried to illegally enter the Islamic state. The agency later withdrew the story, which had given no source.
Iran's Arabic-language television Al-Alam quoted Iran's Revolutionary Guards, in charge of Iran's border security, as denying that any such incident had happened in the southeastern province of Sistan-Baluchestan.
In Washington, a US Defense Department spokesman said: "Reports by state-run Iranian media that seven US soldiers were detained after crossing into Iran are false."
A spokesman for NATO-led forces in Afghanistan, US Captain Ryan Donald, said no US soldiers were missing.
The Fars report came at a time of high tension between Tehran and Washington, which have lacked diplomatic relations since Iran's 1979 Islamic Revolution and are at odds over many issues particularly Iran's disputed nuclear program.
Iran on Tuesday freed one of three Americans held for over a year ago for alleged spying. Sara Shourd was detained near Iran's border with Iraq in late July 2009 along with two male companions, Shane Bauer and Josh Fattal. Their families say the three were on a mountain hike in northern Iraq at the time.
President Mahmoud Ahmadinejad arrived in New York on Sunday to attend the annual UN General Assembly meeting.
The UN Security Council has imposed four rounds of sanctions against Tehran for refusing to suspend sensitive parts of its nuclear work that the United States and its allies suspect is aimed at developing weapons.
Iran denies this and refuses to halt its uranium enrichment program.
