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

A 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 and will not deploy outside the camp,” the statement said.

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

The Western deployment comes as Sangin, a Taliban stronghold for years, appeared close to a complete takeover by the Taliban after weeks of heavy fighting there and in neighboring districts.