WASHINGTON (AFP) - A motorcycle-riding Taleban suicide bomber killed six US soldiers near Kabul Monday, in a brazen attack as the resurgent militant group battled to seize a key southern district in Afghanistan's opium-growing heartland.
The bombing during a joint patrol with Afghan forces near Bagram, the largest US military base in Afghanistan, marks one of the deadliest attacks on foreign troops in the country this year.
The Taleban claimed responsibility for the assault, which underscores a worsening security situation a year after Nato formally ended its combat operations in Afghanistan.
"Six (Nato) service members died as a result of a vehicle-borne improvised explosive device attack," the US-led coalition said in a statement, adding that three others were wounded.
A US official said all six soldiers killed were Americans.
The attack came as Taleban insurgents in Helmand were closing in on the strategic district of Sangin, tightening their grip on the volatile southern province.
Local residents reported crippling food shortages in the district, long seen as a hornet's nest of insurgent activity, after the Taleban began storming government buildings on Sunday.
"The Taleban have captured the police headquarters, the governor's office as well as the intelligence agency building in Sangin," deputy Helmand governor Mohammad Jan Rasoolyar said.
"Fighting is escalating in the district," he said, claiming the number of soldiers killed in clashes is "unbelievably high".
Rasoolyar's comments come a day after he posted a desperate plea on Facebook to President Ashraf Ghani, warning the entire province was at risk of falling to the Taleban.
The government in Kabul said reinforcements had been dispatched to Sangin, while denying claims of large casualties and rejecting that the district was at risk of being captured.
But trapped residents told AFP that roads to Sangin had been heavily mined by insurgents and exhausted soldiers besieged in government buildings were begging for food rations.
The grim assessment bore striking similarities to the security situation that led to the brief fall of the northern city of Kunduz in September - the biggest Taleban victory in 14 years of war.
The fall of Helmand would deal another stinging blow to Afghan forces who have struggled to rein in the ascendant insurgency without the full backing of Nato forces.
Sangin, a strategically important district at the centre of Afghanistan's lucrative opium trade, has been the scene of fierce fighting for years between the Taleban and Nato forces.
British troops fought deadly battles in Sangin for four years to little effect, before US marines replaced them in late 2010 and finally pulled out themselves last year.
"The Taleban onslaught in Sangin is not just a military gain but also a huge propaganda score for the insurgents," Kabul-based political analyst Haroon Mir told AFP.
"The British and Americans bled to defend Sangin district and now Afghan forces are suffering the same fate." All but two of Helmand's 14 districts are effectively controlled or heavily contested by Taleban insurgents, officials said.
Insurgents also recently overran Babaji, a suburb of the provincial capital Lashkar Gah, fuelling concern that the city could fall to the insurgents.
Highlighting the gravity of the situation, US special forces have been sent to Helmand in recent weeks to assist Afghan forces, a senior Western official told AFP without offering details.
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.
President Barack Obama in October announced that thousands of US troops would remain in Afghanistan past 2016, backpedalling on previous plans to reduce the force and acknowledging that Afghan forces are not ready to stand alone.