WASHINGTON • Islamic State in Iraq and Syria (ISIS) fighters killed a US Navy Seal after they broke through Kurdish defences in northern Iraq in what military officials described as the militants' biggest offensive in months.

Charlie Keating IV, a grandson of the late Arizona financier Charles H. Keating Jr who was imprisoned in the savings-and-loan crisis of the 1980s, was the third American to die in combat since a US-led coalition began fighting ISIS in 2014.

His death was also the third in combat since the US troop withdrawal from Iraq in 2011. He was 3km to 4.8km behind front lines when he was killed by small-arms fire, US military officials said.

US advisers have been assisting Iraqi security forces and Kurdish peshmerga fighters as they push to reclaim territory from ISIS.

For months, US military officials have said ISIS was on the defensive. But on Tuesday, the group's fighters used Humvees and bulldozers to break through Kurdish positions and enter the town of Teleskof.

A US official said the ISIS assault - in which several peshmerga fighters also died - was an effort to stymie an advance by the Iraqi military on Mosul, about 22km from Teleskof. Mosul, Iraq's second-largest city, has been held by ISIS since 2014.

With the military pushing towards Mosul, officials say such clashes may become more frequent, which could heighten the danger to US troops in Iraq.

While US President Barack Obama has insisted he is not putting combat troops on the ground in Iraq, the definition of combat has become more blurred with each American death.

Obama administration officials took pains to say the latest casualty did not occur on the front line of battle. Still, US Defence Secretary Ash Carter, in Germany on Tuesday, described it as a "combat death".

NEW YORK TIMES