Afghan National Army (ANA) soldiers patrol at an outpost in Helmand province, Afghanistan, in this December 25, 2015 file photo. REUTERS/Abdul Malik/Files

LASHKAR GAH, Afghanistan (Reuters) - The Taliban were threatening on Tuesday to capture three key strategic districts in Afghanistan’s province of Helmand as fierce fighting with government forces stoked fears over the Islamist insurgents’ gains in their traditional heartland.

The arid, semi-desert southern region is a major center of opium cultivation where the Taliban have stepped up pressure on security forces since the withdrawal of international troops from combat last year.

The government has sent reinforcements from Kabul to protect the districts of Gereshk, Sangin, and Marjah around the provincial capital Lashkar Gah, Helmand’s police chief, Abdul Rahman Sarjang, said. Some of the fighting is hundreds of meters (yards) from the main highway linking the major southern city of Kandahar with the western city of Herat, he said. “The Taliban are putting all their efforts in,” Sarjang said. “We have received some reinforcements and we will get more soon.” The biggest problem for security forces is hit-and-run attacks by the Taliban, as when reinforcements move to one district they attack another, said Omar Zwak, a spokesman for the provincial governor. “They usually attack the weak places and exploit the gaps in our security,” he said.In a sign of the growing violence engulfing Afghanistan, the Taliban are unleashing major attacks during the winter, a break from the past, when the cold weather typically prompted them to hold off from fighting.

The Taliban fight to take Helmand has echoes of the build-up to September’s attack on the northern city of Kunduz, their biggest success in the 15-year war, with Taliban forces picking off a series of districts around the provincial capital.