A NATO air strike in Afghanistan has killed five US soldiers in an apparent “friendly fire” accident during clashes with insurgents, as troops try to ensure security for the presidential election.

Local police and the Afghan army said that the US troops, as well as one Afghan soldier, were killed in the restive province of Zabul when air support was unleashed to try to beat back Taliban fighters.

Friendly fire incidents have been rare in Afghanistan in recent years, though five Afghan soldiers were killed in a NATO air strike in the eastern province of Logar in March.

“Five American troops were killed yesterday during a security operation in southern Afghanistan,” Pentagon spokesman Rear Admiral John Kirby said in Washington on Tuesday.

“Investigators are looking into the likelihood that friendly fire was the cause.”

Afghanistan is braced for Saturday’s run-off presidential election – which the Taliban have vowed to target – and Monday night’s joint US-Afghan operation was tasked with boosting security ahead of polling day.

US Army soldier Aaron Toppen’s grandmother Betty Winter is comforted. Photo: AAP

“Our forces were jointly engaged in fighting with militants. Foreign forces called in air support and they mistakenly bombed friendly positions,” said Mohsin Khan, spokesman for the Afghan army’s 205 division.

The US-led International Security Assistance Force (ISAF) is winding down operations in Afghanistan after more than a decade of fighting the Taliban, and the number of soldiers on the front line has fallen rapidly in the last year.

Monday’s deaths in Arghandab district, a Taliban stronghold, were the worst single incident for the NATO force since five British soldiers were killed in a helicopter crash on April 26.

All of the 50,000 remaining NATO combat troops are due to leave the country by the end of this year, though a small US deployment will remain until the end of 2016 if a long-delayed deal is signed between Washington and Kabul.