BAGRAM AIR BASE, Afghanistan - The incoming general in charge of U.S. troops along the Pakistan-Afghanistan border vowed yesterday to be careful in the use of air strikes, a contentious issue here because of the civilian casualties they can cause.

Maj. Gen. Curtis Scaparrotti, commander of the 82d Airborne Division, took over command of all troops in 14 provinces in eastern Afghanistan from departing Maj. Gen. Jeffrey Schloesser, commander of the 101st Airborne Division, which has spent the last 15 months in the country.

Scaparrotti took only two questions at a media conference, and was immediately asked about civilian casualties.

"We look very closely at the use of close air support in terms of being deliberate and very precise," he said. "We are here to protect the people of Afghanistan, and we intend to pursue our operations with that first in mind and use CAS [close air support] only where we need to protect our force and complete the mission."

"But we will be careful in its use," he said.

A U.S. defense official said yesterday that the U.S. military's failure to follow tightened rules for aerial strikes likely caused civilian deaths in a May 4 American bombing in western Afghanistan. The finding comes from an internal review of the incident, said the official, who spoke on grounds of anonymity because the investigation was not complete.

The Afghan government says 140 civilians died in the May 4 battle in Farah province, while American commanders say video evidence recorded by fighter jets and the account of the ground commander suggest no more than 30 civilians were killed, as well as 60 to 65 Taliban.

In Geneva, Switzerland, U.N. human-rights investigator Philip Alston said that about two-thirds of those killed May 4 appeared to be civilians, citing studies by the United Nations, the Afghan Independent Human Rights Commission, and others.

"There's no disagreement that a very significant number of civilians were killed," Alston told journalists.

The new U.S. ambassador, Karl Eikenberry, a former three-star general who has served in Afghanistan, said yesterday that "it will prove difficult to avoid all civilian casualties" in future battles. He spoke at the opening of a U.S.-funded civilian airport in Lashkar Gah, the capital of Helmand, where newly arriving Marines are deploying.

At the main U.S. base at Bagram before several hundred troops, the departing commander of U.S. and NATO forces in Afghanistan, Gen. David McKiernan, said historians would point to the 101st's 15 months in Afghanistan "as a key shift in momentum" in the almost eight-year war.