KABUL, Afghanistan - A suicide attacker rammed an explosives-laden motorcycle into a joint NATO-Afghan patrol Monday, killing six American troops in the deadliest attack on international forces since August. Two U.S. troops and an Afghan were wounded.

The attack happened as Taliban fighters overran a strategic district in southern Helmand province, the scene of some of the deadliest fighting between the Taliban and international combat forces before the 2014 withdrawal, adding weight to a Pentagon assessment that the insurgency is gaining strength.

The soldiers were targeted as they moved through a village near Bagram Airfield, the largest U.S. military facility in Afghanistan, NATO and Afghan officials said. The Taliban claimed responsibility.

A U.S. official confirmed that six American troops were killed and two wounded. An Afghan was also wounded. The official was not authorized to discuss the incident publicly and spoke on condition of anonymity.

"Our heartfelt sympathies go out to the families and friends of those affected in this tragic incident, especially during this holiday season," Army Brig. Gen. William Shoffner, head of public affairs at NATO's Resolute Support base in the Afghan capital of Kabul, said in a statement.

In Washington, White House press secretary Josh Earnest said that the nation's thoughts and prayers are with the victims, their families, and their loved ones and that the United States will continue to work jointly with Afghans to promote peace and stability in their country.

Secretary of Defense Ash Carter in a statement called the attack "a painful reminder of the dangers our troops face every day in Afghanistan."

It was the deadliest attack on foreign troops in four months.

On Aug. 22, three American contractors with the Resolute Support base were killed in a suicide attack in Kabul. On Aug. 7 and 8, Kabul was the scene of three insurgent attacks within 24 hours that left at least 35 people dead. One of the attacks, on a U.S. special operations forces base outside Kabul, killed one U.S. soldier and eight Afghan contractors.

In the year since the international draw-down, the Taliban insurgency has intensified. Although the combat mission ended last year, about 9,800 U.S. troops and almost 4,000 NATO forces remain in Afghanistan. They have a mandate to "train, assist, and advise" their Afghan counterparts, who are now effectively fighting a battle-hardened Taliban alone.