A resurgent Taliban has claimed responsibility for a suicide bomb attack near Bagram air base that killed six U.S. troops, bringing the total number of American fatalities in the 14-year-old Afghanistan war to at least 2,235.

The six American troops were killed when a suicide bomber on a motorbike struck a joint US-Afghan patrol in southern Afghanistan’s Helmand province, U.S. officials have confirmed on condition of anonymity because the Pentagon has not officially announced the fatalities.

Two U.S. troops and an Afghan were also injured in the attack. The Taliban claimed responsibility for the attack on Twitter, inaccurately claiming 19 U.S. soldiers had been killed.

Bagram, about 25 miles north of the Afghan capital, Kabul, is the largest American facility for the nearly 10,000 U.S. troops that remain in Afghanistan.

“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 prior to the 2014 withdrawal, adding weight to Pentagon predictions that the insurgency is gaining strength,” reports The Associated Press (AP).

AP learned from NATO and Afghan officials that “the soldiers were targeted as they moved through a village near Bagram Airfield, the largest U.S. military facility in Afghanistan.”

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

A Pentagon report released last week warned that the security in Afghanistan has deteriorated in the second half of 2015, noting an increase in the number of effective strikes by Taliban insurgents and a growing Islamic State (ISIS/ISIL) presence.

Taliban-linked violence has increased since President Obama and NATO ended their combat mission in December 2014 and transitioned to a train, advise, and assist (TAA) role.

“The Taliban-led insurgency has likely been emboldened by the coalition’s transition from direct combat operations to a TAA role and the accompanying reduction of coalition combat enablers,” reports the Pentagon. “As a result, the Taliban will continue to test the ANDSF [Afghan National Defense and Security Forces] aggressively in 2016.”

Citing conversations he had with U.S. military officials during his recent visit to Afghanistan, Rep. Robert Pittenger (R-NC), chairman of the Congressional Task Force on Terrorism and Unconventional Warfare, told Breitbart News that the Afghan security forces are not ready to take on the enemy on their own.

The North Carolina Republican said that conversation he had with Gen. John Campbell, the top commander of U.S. and NATO forces in Afghanistan, led him to conclude that the general needs more American troops to combat the enemy.

Currently, there are nearly 10,000 U.S. troops in Afghanistan. President Obama plans to reduce that number to 5,500 by the beginning 2017.

Excluding Monday’s fatalities, the most recent Pentagon tally shows that 2,229 U.S. service members have been killed in Afghanistan since the war started in October 2001.

According to an analysis by Breitbart News, an estimated 75 percent of all U.S. military deaths in Afghanistan have occurred under President Obama’s watch.

“It was the deadliest attack on foreign troops in four months. On Aug. 22, three American contractors with the RS base were killed in a suicide attack in Kabul,” notes AP, referring to Monday’s attack. “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 civilian contractors.”

“Monday’s attack came as Taliban fighters and government forces battled for control of a strategic district in the southern province of Helmand after it was overrun by insurgents, delivering a serious blow to the government’s thinly spread and exhausted forces,” it adds. “Mohammad Jan Rasulyar, Helmand’s deputy governor, said insurgents took control of Sangin district late Sunday.”

Helmand is where most of the world’s opium is produced, making it an important source of revenue for the Taliban.

The province, which borders Pakistan, has been one of the deadliest regions for U.S. forces throughout the Afghanistan war.