TEHRAN (Reuters) - Iranian border guards detained seven U.S. troops as they tried to illegally enter the Islamic state, the semi-official Fars news agency reported on Sunday, without giving a source.

“Recently seven American troops were detained by Iranian guards in a southeastern province of the country ... Two Iranians accompanying the troops were also arrested,” said Fars.

No other news agency reported the incident, which Fars said occurred in Sistan-Baluchestan, an impoverished province that borders Pakistan and Afghanistan.

If confirmed, the incident would aggravate 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.