Top U.S. officer warns Afghan war will get worse

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.