Afghan and U.S. forces battled insurgents in central Wardak province Sunday, near the site where a U.S. Army helicopter went down a day earlier, killing 30 U.S. troops, including special force Navy SEALs. Seven Afghan commandos and an interpreter also died in the crash.

The fighting was ongoing as NATO teams tried to recover the wreckage of the twin rotor Chinook transport helicopter from the remote Tagab Valley.

It was the worst loss of life suffered by U.S. forces in a single incident since the start of the Afghan war in late 2001.

NATO said Sunday it is investigating claims that Taliban insurgents shot down the aircraft. Local Afghan government forces said the helicopter was hit by a rocket following a U.S.-led raid on a Taliban compound. Eight Taliban fighters also were killed in the battle.

In other violence, NATO said four of its service members were killed Sunday in two separate insurgent attacks in southern and northeastern Afghanistan.

Also Sunday, Afghan President Hamid Karzai called his U.S. counterpart, Barack Obama, to reiterate his condolences for the tragic loss of 30 American troops in Wardak. A White House statement said Mr. Obama noted the extraordinary service of the Americans who gave their lives, and expressed his condolences for the Afghans who died serving with them.

The two sides also reaffirmed their commitment to the mission in Afghanistan, which they said was critical to the security of both countries.