KABUL,  (Reuters) – More NATO troops will die in  Afghanistan as violence mounts over the summer, but  Washington’s goal of turning the tide against the insurgency by  year’s end is within reach, the top U.S. military officer said  yesterday.

The remarks by Chairman of the Joint Chiefs of Staff,  Admiral Mike Mullen, on a visit to the country, came as the  Taliban said they were holding captive one of two U.S.  servicemen who strayed into insurgent territory, and that the  other had been killed.

It also comes less than a week since a major international  conference in Kabul agreed that the Afghan government should  aim to take responsibility for security in all parts of the  country by 2014.

Mullen, who called the troops’ disappearance an “unusual  circumstance,” said there would be more violent incidents to  come, but the U.S. military was doing everything possible to  find the missing men, who were both from the Navy.

A spokesman for the NATO-led force declined to comment on  the Taliban’s announcement it was holding one of the men.       The Navy described both men as still missing.

“Forces on the ground in Afghanistan are doing everything  they can to locate and safely return our missing shipmates,”  Admiral Gary Roughead, Chief of Naval Operations, said in a  statement.

The Afghan government said on Sunday it was checking  reports from villagers that civilians had been killed in a raid  by foreign forces in Sangin, in southern Helmand province, on  Friday.

The NATO-led force said it was aware of reports of the  incident and was investigating, but would not comment further  until further details were available. Such incidents have  triggered outrage in the past among the population against the  international troops whose mission is to protect them.