The International Committee of the Red Cross (ICRC) suspended operations in Afghanistan on Wednesday after gunmen killed six employees helping deliver emergency relief to a remote northern region hit by heavy snow storms.
The governor of Jowzjan province said the aid convoy was attacked by suspected Islamic State gunmen. The head of the ICRC called the incident the "worst attack against us" in 20 years, but the charity said it did not know who was responsible.
A search operation was underway to find two charity workers who were still missing late on Wednesday night.
"As we speak our operations are on hold indeed, because we need to understand what exactly happened before we can hopefully resume our operations," ICRC director of operations Dominik Stillhart told Reuters in Geneva.
Afghanistan is the ICRC's fourth largest humanitarian program in the world, Stillhart said, and the attack follows a warning by the charity last month that mounting security issues made it perilous to deliver aid to large swathes of the country.
A massive snowstorm dumped as much as two meters (6.5 feet) of snow on areas of Afghanistan over the weekend, according to officials, killing more than 100 people.
Lotfullah Azizi, the Jowzjan provincial governor, said the aid workers were carrying livestock supplies to areas badly affected by the storm.
"Daesh is very active in that area," he said, using an alternative name for Islamic State, which has made limited inroads in Afghanistan but has carried out increasingly deadly attacks.
The ICRC team included three drivers and five field officers. Jowzjan police chief Rahmatullah Turkistani said the workers' bodies had been taken to the provincial capital.
"These staff members were simply doing their duty, selflessly trying to help and support the local community," ICRC president Peter Maurer said.
Taliban spokesman Zabiullah Mujahid said his group was not involved in the attack and promised that Taliban members would "put all their efforts into finding the perpetrators".
Gunmen in northern Afghanistan kidnapped a Spanish ICRC employee in mid December, releasing him nearly a month later.
That staff member had been traveling with three Afghan colleagues between Mazar-i-Sharif and Kunduz when gunmen stopped their vehicles. The local staff were immediately released.
In a summary of its work in Afghanistan last year, the ICRC said increasing security issues hampered the provision of aid to many parts of the country.
"Despite it all, the ICRC has remained true to its commitment to the people of Afghanistan, as it has throughout the last 30 years of its continuous presence in the country," the statement said.
Besides determining the operational impact of the attack, Stillhart said ICRC would pause its programs out of respect for the slain aid workers.
"We also need and want to mark what is a horrible incident, which came as a huge shock for all our staff, first and foremost in Afghanistan but also to respect the families," he said.