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

Maj.-Gen. Curtis Scaparrotti, commander of the 82nd Airborne Division, took over command of all troops in 14 provinces in eastern Afghanistan from outgoing 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," Scaparrotti 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. defence official said Wednesday 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, UN human rights investigator Philip Alston said 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 Wednesday that "it will prove difficult to avoid all civilian casualties" in upcoming 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 now deploying.

At a ceremony at the main U.S. base at Bagram before several hundred troops, the outgoing 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.

McKiernan told the crowd that the 101st targeted 123 insurgent leaders or facilities. Schloesser said more than 180 U.S. and coalition troops died and around 810 had been wounded in eastern Afghanistan in the previous 15 months.

McKiernan, who has served as U.S. and NATO commander for one year, is soon to be replaced by Lt.-Gen. Stanley McChrystal after having his two-year deployment cut short by Defence Secretary Robert Gates.

Schloesser, calling McKiernan "an American patriot," said the four-star general has "done more for this country and in a way done more for the United States of America, as well as all the countries in ISAF, and it will be written in history." The tribute brought a long round of applause, but McKiernan did little to acknowledge it.

Violence in Afghanistan has ratcheted up the last several weeks, a precursor of the battles likely to take place this summer as 21,000 additional U.S. troops join the fight.

On Wednesday, a suicide bomber on a motorbike attacked a convoy of Afghan security guards ferrying supplies to international troops, killing five of them. The attack in Spin Boldak district of Khandahar occurred about two kilometres from the Pakistan border, said district police chief Safiullah Khan.