(Adds Pentagon statement, abduction of 35 professors)
KABUL, June 10 (Reuters) - Five U.S. servicemen were killed in southern Afghanistan in a friendly-fire air strike during a security operation, Afghan police and the Pentagon said on Tuesday, days before a run-off round in the country's presidential election.
The men died on Monday in Zabul province's Arghandab district when their unit, part of the NATO-led International Security Assistance Force (ISAF), clashed with insurgents.
Local police chief Ghulam Sakhi Roghlewanai said: "The five killed were American soldiers who just returned from an operation when they were hit.
"ISAF troops were returning to their bases after an operation when they were ambushed by the insurgents. The air strike mistakenly hit their own forces and killed the soldiers."
A Pentagon statement said investigators were "looking into the likelihood that friendly fire was the cause. Our thoughts and prayers are with the families of these fallen."
A spokesman for the Islamist Taliban, Qari Yousuf Ahmadi, said insurgents had been attacking the foreign forces when the helicopters intervened and accidentally killed their own troops.
Taliban insurgents, meanhwhile, kidnapped 35 professors from Kandahar University after stopping their van on the highway linking the southern province and Kabul, a spokesman for provincial governor said.
"The professors were on their way to the capital when they were abducted and tribal elders are now involved in negotiating with the Taliban," Dawa Khan Minapal said by telephone.
The Taliban, removed from power by a U.S.-led drive into Afghanistan after the September 11, 2001 attacks, is on an offensive ahead of the planned withdrawal of most foreign troops by the end of 2014.
Security is being ramped up in Afghanistan ahead of Saturday's run-off vote to replace President Hamid Karzai.
The poll pits Abdullah Abdullah, a former leader of the opposition to the Taliban, against former Finance Minister Ashraf Ghani. (Reporting by Hamid Shalizi in Kabul, Sarwar Amani in Kandahar, Writing by Praveen Menon; Editing by Ron Popeski)