At least 11 Afghan soldiers were killed in a Taliban ambush in western Afghanistan, officials said Monday, as security forces face their first fighting season without NATO combat support.

"The Taliban ambushed a convoy of soldiers in Karukh district of Herat, killing 11 Afghan army soldiers last evening. The soldiers were riding in pickup trucks," Ehsanullah Hayat, spokesman for the governor of the western province of Herat, told AFP.

Najibullah Najibi, a spokesman for the army in western Afghanistan, confirmed the attack and said four soldiers were also wounded.

There was no immediate claim of responsibility, but Taliban militants have increased their attacks on Afghan security forces since launching their annual spring offensive.

The Taliban launched their offensive, named "Azm" (determination), in late April, vowing nationwide attacks in what is expected to be the bloodiest summer in a decade.

Afghan security forces, stretched on multiple fronts and facing record casualties, are struggling to rein in the militants even as the government makes repeated efforts to jump-start peace negotiations.

NATO's combat mission formally ended in December after 13 years, but a small follow-up foreign force has stayed on to train and support local security forces.