Gunmen opened fire on people in bed at a camp in Afghanistan, killing 10 landmine-clearance workers.
The victims were staff of a humanitarian organisation, the Halo Trust, which said the attack was “genuinely horrific”.
Afghan officials blamed the killings on Taliban insurgents.
“The Taliban brought them into one room and opened fire on them,” provincial police spokesman Jawed Basharat said.
But the Halo Trust did not attribute blame, saying only that an “unknown armed group” carried it out.
The insurgents, fighting to overthrow Afghanistan’s foreign-backed government, denied involvement in the attack on the camp in the northern province of Baghlan, where fighting has been heavy in recent weeks.
The United Nations called for a full investigation to “ensure that those responsible for this horrendous attack are held accountable and brought to justice,” said UN spokesman Stephane Dujarric.
The Halo Trust, the largest demining organisation in Afghanistan, said the gunmen killed 10 of its local staff and wounded 16.
James Cowan, of the trust, said the attackers went bed-to-bed murdering staff.
“This is the most serious incident the trust has endured since it came into being in Afghanistan in 1988,” he said.
Paying tribute to the dead, he said: “Each one was a member of a family – was a father, or a brother or a son, and the hole they’ve left was huge.
“What took place was genuinely horrific.”
The trust’s 2,500 staff in the country, all Afghans, risk their lives clearing landmines every day, he said.
After decades of conflict, Afghanistan is strewn with mines and unexploded ordnance and agencies have been working to clear them in the years since the Taliban were ousted in 2001.
An official in the area said most of the surviving workers fled to nearby villages after the attack.
A Taliban spokesman denied involvement in the killings. But a senior government official in the capital Kabul dismissed that, saying: “This was clearly execution by the Taliban.”
The Taliban often attack demining workers because, government officials say, the workers often help to defuse roadside bombs that the insurgents have planted.
Violence has increased across Afghanistan since the US announced plans in April to pull out all of its troops by 11 September.
The Taliban are fighting government troops in 26 out of 34 provinces.
Mr Cowan pledged the Halo Trust would stay in the country.
Additional reporting by Reuters