KABUL, Afghanistan - The U.S. military installed a new commander in Afghanistan today, a fresh Army general who pledged to be "relentless" in combating insurgents still dogging the country more than three years after the fall of the Taliban.

Lt. Gen. Karl Eikenberry took over from fellow three-star David Barno in a ceremony at the U.S. military headquarters in the Afghan capital, Kabul, before guests including Gen. John Abizaid, the chief of the U.S. Central Command.

Barno departs after more than 18 months in which the military has shifted its focus from the fruitless search for fugitives such as Osama bin Laden and Taliban leader Mullah Omar toward stabilizing a new strategic ally through reconstruction and rebuilding its feeble government.

The 18,000-strong coalition helped protect landmark elections won by U.S.-backed President Hamid Karzai last year. Afghans are to vote for a new parliament in September to complete the country's democratic rebirth.

But the new commander must also grapple with a recent surge of violence that has killed several civilians as well as dozens of rebels and two coalition soldiers and undermined assertions by Barno and others that the insurgency is weakening.

"We will continue to prosecute the war against terror in partnership with the Islamic government of Afghanistan and will be relentless as we move forward," Eikenberry said in his inaugural speech.