Iran denied on Sunday that border guards had detained seven U.S. troops, calling the report "unfounded", the state-run English language Press TV said.
The country's Arabic language television al-Alam also 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.
A senior defense official told NBC News that no U.S. soldiers were missing in the area near the Iranian border. The official described it as a "false report."
Earlier, the semi-official Fars news agency had reported that the American troops had been arrested while trying to illegally enter the Islamic state.
"Recently seven American troops were detained by Iranian guards in a southeastern province of the country," Fars said. "Two Iranians accompanying the troops were also arrested."
Fars didn't cite a source for its report.
Iran on Tuesday freed one of three Americans held for over a year ago for alleged spying. Sarah 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.