(CNN) -- Two U.S. Marines died Monday during a "hostile incident" in southern Afghanistan, the U.S. military said Tuesday.
U.S. Marines walk through a field on patrol on July 13, 2009, in Mian Poshteh, Afghanistan .
No further details were immediately available in their deaths.
"We deeply mourn the loss of these determined service members, who died in combat defending our freedom and the just cause of Afghans," said Brig. Gen. Eric Tremblay, spokesman for NATO's International Security Assistance Force.
"This is a moment of great sorrow for these members' families and friends, and I extend my deepest condolences to them during this difficult time."
The deaths come as almost 4,000 Marines and sailors, along with several hundred Afghan security forces, are working to clear Taliban militants from population centers in the Helmand river valley, in the south of the country.
British forces launched a similar offensive in the province in late June.
The push, called Operation Khanjar, is targeting militants in Helmand province, a Taliban stronghold and poppy-growing region.
The forces are trying to gain and hold ground in the perilous region ahead of national elections this August.