Four US soldiers have been killed in action in southern Afghanistan, the US military says.
US troops are fighting an insurgency in the south and east of the country
The deaths occurred in the province of Zabul, which has seen regular attacks by Islamic militants.
"Four US service members assigned to the Combined Joint Special Operations Task Force - Afghanistan were killed in action today... ," the statement said.
"Names will not be released until notification of next of kin is complete," it added.
The US military says the four died when an explosive device detonated under the military vehicle - a Humvee - that they were travelling in approximately 30 kilometres east of Qalat.
The deaths represent one of the biggest losses for American forces since they began operations in Afghanistan more than two years ago.
The attacks have been blamed on remnants of the former Taleban regime.
The US military acknowledges that an insurgency in the south and east of the country has gathered pace - more than 700 people are said to have died in violence in Afghanistan since last August.
Suspected Taleban militants have attacked Afghan government targets in the southern province of Helmand, killing at least seven Afghan soldiers.
Local officials said four Taleban fighters were also killed and a number captured during the clashes at Musa Qala early on Sunday.
Correspondents say there were several apparently co-ordinated raids on government buildings and military and police posts.