The Taliban are pressing ahead with their advances in southern Afghanistan, capturing nine out of 10 districts of the Helmand provincial capital, residents and officials say.
Afghan government forces launched airstrikes, backed by the US, in a desperate effort to defend the city of Lashkar Gah.
The fall of Lashkar Gah would be a major turning point in the offensive the Taliban have waged over the past months as US and NATO forces complete their pullout from the war-torn country.
It would also be the first provincial capital captured by the Taliban in years.
Residents of the city, speaking to The Associated Press over the telephone, said the fighting has them trapped, hunkered down inside their homes and unable to step out for basic supplies.
They said Taliban fighters were out openly in the streets, and that all but one Lashkar Gah district was under Taliban control.
Elite commando units were dispatched from Kabul to aid Afghan forces as the government held on to key government buildings, including the local police and army headquarters.
Majid Akhund, deputy chairman of the Helmand provincial council, confirmed that the Taliban control nine Lashkar Gah districts and also the city's TV and radio station, which had both gone off the air.
The Afghan forces commander for Helmand, General Sami Sadat, in an audio message shared with journalists Tuesday urged residents in neighbourhoods captured by the Taliban to evacuate immediately. The message was an indication more airstrikes were planned.
Lashkar Gah is one of three provincial capitals under siege by the Taliban as they stepped up their onslaught against government forces.
Over the past weeks, the Taliban have captured several lucrative border crossings with Iran, Pakistan and Tajikistan.
Most recently, the Taliban have turned their guns on provincial capitals as the withdrawal of the US and NATO forces is now more than 95 per cent complete.
The two other provincial capitals under siege are in the neighbouring province of Kandahar, also in the south, and in western Herat province.
Afghan President Ashraf Ghani on Monday blamed the hasty withdrawal of US and NATO troops for the deteriorating security situation, while analysts say deep corruption and poor training has left Afghan forces overwhelmed, leaving the elite commando units as the only bulwark against the advancing Taliban.
The US and other world leaders have warned the Taliban against a military takeover of Afghanistan, saying they would become an international pariah again if they tried to take power by force.
When they last ruled Afghanistan, the Taliban were recognised by only three countries - Pakistan, Saudi Arabia and the United Arab Emirates.
Australian Associated Press