Reuters/Lashkar Gah, Afghanistan

Taliban forces advanced on the capital of the volatile southern Afghan province of Helmand on Tuesday amid fierce fighting with government forces that threatened to cut off a major highway and prompted many families to flee.

The fighting near the town of Lashkar Gah comes three weeks after the Taliban won its biggest victory in the 14-year war, capturing the northern town of Kunduz and holding the city centre for three days before government forces regained control.

"Helmand's capital appears to be under serious military pressure," a Western official said. "We're hearing reports about civilians fleeing in large numbers."

Provincial Governor Mirza Khan Rahimi said heavy fighting had been going on for two days in the district of Gereshk to the north of the city. The fighting has threatened Highway One, the main transport artery linking the major southern city of Kandahar with Herat.

Government officials said Lashkar Gah would not fall but one security source said the town of Baba Ji, to the north of the provincial capital had fallen and the Taliban itself said it was making major advances.

"The fighting is ongoing in full force and we have killed 25 government forces and seized a number of weapons," said spokesman Qari Yousuf Ahmadi.

Helmand province is one of the world's biggest centres of opium cultivation, with a complex mix of warring tribal groups and Taliban insurgents creating a chronic problem for the Western-backed government.

Fall of Kunduz

The fall of Kunduz at the end of last month came as a profound shock in Afghanistan, piling pressure on the government of President Ashraf Ghani and raising questions over the capacity of security forces to maintain stability.

Afghan government forces, which have taken on the bulk of fighting since international troops ended most combat operations last year, have struggled to contain the Taliban, which has stretched the army in a series of operations across the country.

Having briefly taken Kunduz at the end of September, the Taliban last week threatened the city of Ghazni, southwest of Kabul, cutting off the main route between Kabul and Kandahar for three days.

Earlier this week, they were also reported to have overrun the district of Ghormach in Faryab province on the northern border with Turkmenistan.

The deteriorating security situation prompted US President Barack Obama last week to delay plans to pull American forces out of the country next year, with at least 5,500 troops now due to remain after 2017.