Sangin district has fallen to the Taliban just a day after Helmand province's deputy governor used Facebook to plead with the Afghan president for help in holding off the armed group.
Sangin fell to the Taliban after hours of fierce clashes that killed more than 90 soldiers in two days, an Afghan police spokesperson told Al Jazeera, with the Taliban taking over police and military installations.
The Taliban also confirmed the siege to Al Jazeera.
Al Jazeera's Abdullah Shahood, reporting from the capital Kabul, said fighting was ongoing.
"The special forces unit has arrived and is planning a massive operation led by the Afghan forces," he said. "Some families have been displaced because of the clashes and many are trying to leave."
I cannot be silent any more ... as Helmand stands on the brink. Ninety men have been killed in Gereshk and Sangin districts in the last two days.
Afghanistan's chief executive, Abdullah Abdullah, told a press conference on Monday that "an urgent meeting will happen soon to take immediate security action in Helmand".
"The action will repel enemy attack," he said.
Helmand's deputy governor, Mohammad Jan Rasoolyar, in a post on Facebook on Sunday, asked President Ashraf Ghani for urgent intervention to save a province that British and US forces struggled for years to defend.
"I know that bringing up this issue on social media will make you very angry," Rasoolyar wrote in the post, which was addressed to Ghani.
"But I cannot be silent any more ... as Helmand stands on the brink. Ninety men have been killed in Gereshk and Sangin districts in the last two days."
Local officials backed Rasoolyar's assertions, saying the Taliban were making steady gains in districts such as Sangin.
"These clashes between the Taliban and Afghan forces have been going on for many days now. The Taliban are not far from taking over the entire province as we see," Ali Ahmad, a Lashkar Gah resident, told Al Jazeera.
"The government is not taking necessary actions at all. If they did ahead of time, the situation would not have been this bad."
Responding to the post, deputy presidential spokesman Sayed Zafar Hashemi said the "president is aware of the Facebook post and is taking immediate necessary actions".
The northern city of Kunduz briefly fell to the Taliban in September - the biggest victory for the group in 14 years of war. The fall of Helmand would deal another stinging blow to the country's NATO-backed forces as they struggle to rein in unrest.
This month marks a year since the US-led NATO mission in Afghanistan transitioned into an Afghan-led operation, with allied nations assisting in training local forces.
On Monday, a suicide bomber attacked a joint Afghan-NATO convoy near Bagram airbase near Kabul, killing six foreign soldiers and wounding three others. The Taliban claimed responsibility.
US President Barack Obama announced in October that thousands of US troops would remain in Afghanistan past 2016, back-pedalling on previous plans to shrink the force and acknowledging that Afghan forces were not ready to stand alone.
Al Jazeera's Shahood noted the Taliban has increasingly launched attacks in southern Helmand.
"After the NATO withdrawal, Helmand has seen a worsening security situation. The Taliban have increasingly stepped up their offensives during the winter season," said Shahood.
"This is now a big challenge for Afghan forces after the NATO withdrawal because they're lacking much-needed air support."
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Source: Al Jazeera and agencies