U.S. soldiers are shown in Afghanistan as the sun sets behind them. (Photo released by the U.S. military)

The U.S.-led military coalition in Afghanistan suffered one of its worst days in 2015 on Monday, as six U.S. service members were killed and three more people were wounded when a motorcycle laden with explosives detonated near them in Parwan province in an apparent suicide attack.

A U.S. military official in Afghanistan confirmed that the slain troops were all Americans. The incident occurred north of the capital, Kabul, at about 1:30 p.m. in a province best known for Bagram air base, a sprawling military complex from which U.S. forces fly F-16 fighter jets and other aircraft. While the U.S. military no longer carries out offensive operations in Afghanistan following the end of its formal combat mission, it patrols around the base daily to protect it against the Taliban and other enemy groups.

Three Afghan police officers also were wounded in the blast, an Afghan official said.

“We’re deeply saddened by this loss,” said Brig. Gen. Wilson A. Shoffner, the top U.S. military spokesman in Afghanistan. Gen. John Campbell, the U.S. commander in Afghanistan, and the rest of the military coalition said that “our heartfelt sympathies go out to the families and friends of those affected in this tragic incident, especially during this holiday season,” Shoffner added.

[In Afghanistan, a series of attacks on Americans during ‘non-combat’ operations]

In emailed statements and tweets Monday, the Taliban claimed responsibility for the suicide attack on American soldiers near Bagram and said its forces had successfully overtaken the strategic Sangin district of Helmand province in the south.

A statement from the insurgents said a “self-sacrificing attacker” named Zahidullah, riding a motorbike, targeted American soldiers patrolling in Bagram. The statement also said that 19 American “occupiers” were killed and “a huge number” wounded. The Taliban often exaggerates enemy casualty numbers.

Following several days of intense fighting in Helmand, Taliban spokesman Zabiullah Mujahid tweeted Monday night that its fighters had “conquered” the Sangin bazaar and police headquarters and were entering the district center. Muhajid also said that 13 Afghan forces were killed in Marja, another critical district of Helmand.

But spokesmen for the Afghan defense and interior ministries asserted Monday that Sangin had not fallen to the Islamist militants, although they acknowledged that fighting was taking place in many areas of Helmand.

One day earlier, officials in Helmand had asked President Ashraf Ghani to help prevent the province from falling to the Taliban. The intensifying battles in Helmand, as well as a series of aggressive Taliban attacks in other provinces this month, have reinforced public fears of a nationwide Taliban takeover.

Taliban forces have been making steady advances across Afghanistan since they seized the northern city of Kunduz and held it for the first two weeks of October, destroying numerous buildings and sending residents fleeing. During the fighting, a mistaken U.S. airstrike on a hospital there, which killed numerous patients and staff, added to a sense of chaos and poor coordination between Afghan troops and their U.S. military backers trying to drive out the Islamist fighters.

Since then, the Taliban has staged a range of attacks in various parts of the country, including a deadly siege at the fortified Kandahar airport and military base that left 50 people dead, the beheading of seven civilians in Zabul province, and a suicide bombing at a guest house in Kabul that badly damaged the Spanish Embassy.

At about 9 p.m. Sunday, Kabul was rocked by at least two loud bomb explosions in the heavily guarded diplomatic zone that also includes NATO military facilities. There were no immediate reports of casualties, but sirens wailed for many minutes afterward and English-language warnings echoed from loudspeakers. One warned people to “seek hard shelter.”

With Afghanistan still dangerous, U.S. forces minimize travel by road as much as possible to avoid improvised explosive devices, favoring helicopter travel instead.

But some service members still work off the base, carrying out security patrols, route clearance and other missions. One unit doing so from Bagram is Task Force Buffalo, a U.S. Army-led unit that includes U.S. Marines and soldiers from the United States, the Czech Republic and the Republic of Georgia.

As of Dec. 18, 14 American troops and one civilian had died in Afghanistan while serving with the coalition this year, with five of them killed in combat, according to Pentagon statistics. An additional 68 were wounded in action. The others who died were killed in aircraft crashes and in a handful of incidents the military refers to as “non-hostile,” which usually refer to some sort of accident or a suicide.

Constable reported from Afghanistan.