LASHKAR GAH, AFGHANISTAN: Taliban militants ambushed and killed around 100 Afghan police and soldiers earlier this week as they tried to retreat, the heaviest losses suffered by government forces during months of fierce clashes near the capital of southern Helmand province.

A suicide attacker rammed an explosives-laden motorcycle into a joint NATO-Afghan patrol Monday killing six American service members, in the deadliest attack on international forces in the country since August. Two additional 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 foreign combat troops withdrew in 2014, adding weight to Pentagon predictions that the insurgency is gaining strength.

The troops 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 for the killings.

Thumbnail image for Taliban close to taking over Helmand: Afghan official

Taliban close to taking over Helmand: Afghan official

District falls to Taliban a day after Helmand deputy governor warned government was in danger of losing key province

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

A New York police officer, Detective Joseph Lemm, was among those killed, said the city's Police Commissioner William Bratton. Lemm served in the U.S. National Guard and, while a member of the police force, he was deployed twice to Afghanistan and once to Iraq. He leaves behind a wife and three children.

It was the largest attack on foreign troops in four months. On Aug. 22, three American contractors with the Resolute Support base were killed in a suicide 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, left one U.S. service member and eight Afghan civilian contractors dead.

In the year since the international drawdown, 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.

Monday's attack came as Taliban gunmen and government forces battled for control of a strategic district in Helmand after it was overrun by insurgents, delivering a serious blow to the government's thinly spread and exhausted forces.

Mohammad Jan Rasulyar, Helmand's deputy governor, said insurgents took control of Sangin district late Sunday.

Rasulyar took the unusual step of alerting Afghan President Ashraf Ghani to the dire security situation and requesting urgent reinforcements through an open letter posted on Facebook on Sunday, saying that he had not been able to make contact with him through other means.

"We had to take to social media to reach you, as Helmand is falling into the hands of the enemy and it requires your immediate attention," Rasulyar wrote in his post.

On Monday, Defense Ministry spokesman Dawlat Waziri said Afghan army commandos and special forces arrived in Sangin to push a counteroffensive. He told reporters the Afghan air force conducted 160 combat and transport flights over Sangin in the past 48 hours.

Helmand is an important region for the Taliban because it produces most of the world's opium, a crop that helps fund the insurgency.

Sangin has bounced in and out of Taliban control for some years, and fighting there has produced some of the most casualties among Afghan and international forces in 14 years of war.

British forces saw intense fighting there at the height of the war in 2006 and 2007. The U.K. lost more than 450 troops during its combat mission in Afghanistan — more than 100 of them in Sangin.

In 2008 a battalion of U.S. Marines arrived in Helmand, followed a year later by the first wave of President Barack Obama's surge effort against the Taliban, consisting of 11,000 Marines who conducted operations across the province.

The head of Helmand's provincial council, Muhammad Kareem Atal, said that about 65 percent of Helmand is now under Taliban control. "In every district either we are stepping back or we are handing territory over to Taliban, but still, until now, no serious action has been taken," he said, referring to a perceived lack of support from the capital.

Districts across Helmand — including Nad Ali, Kajaki, Musa Qala, Naw Zad, Gereshk and Garmser — have been threatened by Taliban takeover in recent months. Insurgents are believed to be dug in on the outskirts of the provincial capital, Lashkar Gah.

Taliban fighters, sometimes working with other insurgent groups like the Islamic Movement of Uzbekistan, have managed to overrun many districts across the country this year and staged a three-day takeover of the major northern city of Kunduz. They rarely hold territory for more than a few hours or days, but the impact on the morale of Afghan forces and civilians is substantial.

Thumbnail image for Kandahar assault casts doubt on Afghan security preparedness

Kandahar assault casts doubt on Afghan security preparedness

Civilians among dozens killed in attack on heavily fortified Kandahar airport, coinciding with peace talks in Pakistan

Atal said more than 2,000 security forces personnel were killed fighting in Helmand in 2015. He said a major reason Afghan forces were losing was the large number of soldiers and police deserting their posts in the face of the Taliban onslaught.

The fighting in Afghanistan has intensified since the announcement in late July that the founder and leader of the Taliban, Mullah Mohammad Omar, had been dead for more than two years. His deputy, Mullah Akhtar Mansoor, succeeded him, causing internal rifts and reducing the likelihood that peace talks with the Afghan government, halted after the announcement of Omar's death, will restart in the foreseeable future.

The expected winter lull in fighting has not yet taken place in the southern warmer provinces. U.S. and Afghan military leaders say they are expecting a hot winter, followed by a tough fight throughout 2016. Faced with a war-hardened insurgency and limited international assistance, government forces are wearing thin.

The Pentagon released a report last week warning that the security situation in Afghanistan would deteriorate, because a "resilient Taliban-led insurgency remains an enduring threat to U.S., coalition and Afghan forces as well as to the Afghan people."