KABUL — Besieged Afghan forces struggled to halt a Taliban takeover of the critical southern district of Sangin Tuesday, and a new deployment of British troops was rushed in to help direct an increasingly pressed battle across the surrounding province of Helmand.

The small contingent of British forces in an advisory role arrived at Camp Shorabak, the largest British military base in Afghanistan before it was handed over to the Afghan forces last year, Britain’s Defense Ministry said in a statement.

“They are not deployed in a combat role,’’ the statement said.

The shift, which Afghan officials said included about 40 people, follows an influx of US Special Operations forces that deployed to Helmand when the provincial capital, Lashkar Gah, was on the verge of falling several weeks ago.

The reinforcements come as Sangin, a fierce Taliban stronghold for years, appeared close to a takeover by the Taliban after weeks of heavy fighting.

Ebadullah Alizai, a member of the provincial council in Helmand, said much of the Sangin district center was under Taliban control, with an Afghan army unit surrounded and in urgent need of reinforcements.

But attempts to get the extra troops to Sangin were slowed by roadside bombs, he said.

Sangin has been one of the deadliest Afghan battlegrounds for British and US troops throughout the war. Within months of the NATO transfer of security control to Afghan forces there, the district came under heavy insurgent pressure.

There are currently about 13,000 foreign troops in Afghanistan, including 9,800 Americans, with a mandate to ‘‘train, assist, and advise’’ their Afghan counterparts. That’s compared to 140,000 foreign troops at the peak of combat operations in 2011.

The developments in Helmand came a day after a Taliban suicide bomber killed six US troops near Bagram Air Field — the deadliest day for American troops in the country since 2013.

The identified victims:

¦ Major Adrianna Vorderbruggen, of Plymouth, Minn., an openly gay officer with the Air Force Office of Special Investigations. She had helped successfully lobby Congress to repeal the “Don’t Ask, Don’t Tell’’ policy on homosexuals in the military, a friend told the Washington Post.

¦ Staff Sergeant Peter Taub, 30, with the Air Force Office of Special Investigations. His mother, Arlene Wagner, said Taub leaves behind a pregnant wife and a 3-year-old daughter.

¦ Staff Sergeant Joseph Lemm, a New York City police detective and member of the Air National Guard who had also served in Iraq. Lemm, who was married and had two children, had worked for the New York City Police Department for 15 years, said Police Commissioner William Bratton.

¦ Staff Sergeant Chester J. McBride, 30, of Statesboro, Ga, with the Air Force Office of Special Investigations.

¦ Staff Sergeant Michael A. Cinco, 28, of Mercedes, Texas, with the Air Force Office of Special Investigations.

¦ Staff Sergeant Louis M. Bonacasa, 31, of Coram, N.Y., with the 105th Security Forces Squadron.

Material from the Washington Post and the Associated Press was used in this report.