The military coalition did not disclose the nationality of the soldiers killed in the attack, which highlights a worsening security situation a year after the NATO combat mission ended.
The Taliban claimed responsibility for the attack near Bagram, the largest US military base in Afghanistan, during a joint patrol of NATO and Afghan forces.
"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.
Local residents reported crippling food shortages in the district, long seen as a hornet's nest of insurgent activity, after the Taliban began storming government buildings yesterday.
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"The Taliban have captured the police headquarters, the governor's office as well as the intelligence agency building in Sangin," deputy Helmand governor Mohammad Jan Rasoolyar told AFP.
"Fighting is escalating in the district," he said, claiming the number of soldiers killed in clashes is "unbelievably high".
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 Taliban 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.
The government in Kabul said reinforcements had been dispatched to Sangin, while strongly denying that the district was at risk of being captured.