KABUL (Reuters) - Taliban fighters have captured the strategic district of Sangin in the southern Afghan province of Helmand after security forces pulled out, leaving the district center to the insurgents, officials said on Thursday.

Helmand, which accounts for the bulk of Afghanistan’s billion dollar opium crop, is already largely in the hands of the Taliban but the capture of Sangin underlines their growing strength in the south.

Scores of American and British soldiers died fighting in Sangin in some of their bloodiest battles following the 2001 U.S.-led military intervention, and the loss of the district underlines the scale of the challenge facing the western-backed Afghan government and its international partners.

Afghan forces have struggled to contain the spreading insurgency since international troops ended combat operations in 2014, leaving them to fight largely alone.

With warmer spring weather beginning, increased fighting is expected across Afghanistan. Security officials warn that 2017 may be even tougher than last year, when both Lashkar Gah, the provincial capital of Helmand, and the northern city of Kunduz came close to falling.

Taliban spokesman Qari Yousuf Ahmadi said Taliban fighters had captured police headquarters and a military base overnight, as well as quantities of military equipment after they were abandoned by retreating government forces.

He said the area had been bombarded by foreign forces following the withdrawal of Afghan troops and police, causing heavy damage to buildings and infrastructure.

The NATO-led Resolute Support mission did not respond to a request for comment about whether they were involved at all in the fighting or bombing of the site.

Omar Zwak, the provincial governor’s spokesman, said security forces had conducted a tactical withdrawal some 3 km (1.8 miles) from the district center to avoid civilian casualties.

Abdul Majid Akhondzada, deputy provincial council chief of Helmand, confirmed that after the retreat, air strikes hit the district to destroy military equipment left behind.

U.S. President Donald Trump is yet to announce a new Afghanistan strategy.

The top U.S. commander in the country, Gen. John Nicholson, said last month that Afghanistan was in a “stalemate” and thousands more international troops would be needed to boost the existing NATO-led training and advisory mission.

According to U.S. estimates, government forces control less than 60 percent of Afghanistan, with almost half the country either contested or under the control of the insurgents, who are seeking to reimpose Islamic law after their 2001 ouster.

However, while the Taliban control large areas of the countryside, they have so far failed to capture and hold any provincial capital for more than brief periods.

In Kunduz, which briefly fell to the Taliban in 2015, the insurgents control areas around the city and government rule is precarious. On Thursday, in the latest insider attack, a rogue local policeman killed nine of his colleagues at a security outpost, officials said.

Heavy fighting has also been reported near Tarin Kot, the provincial capital of Uruzgan in south-central Afghanistan, which almost fell to Taliban fighters last year.