Dozens of Afghan civilians were killed on Tuesday when a suicide bomber detonated in a wedding hall where Islamic scholars were celebrating the Prophet Mohammed's birthday in Kabul.
As many as 50 people were killed in the blast in Afghanistan's capital and at least 70 people have been injured, officials said.
No group has yet taken responsibility for the attack.
"A suicide bomber detonated his explosives inside a wedding hall where Islamic religious scholars had gathered to commemorate the birthday of Prophet Mohammed," Najib Danish, a spokesman for the interior ministry, told Reuters.
The United Nations Assistance Mission in Afghanistan said it was shocked by the attack on a holy day.
"UNAMA outraged by Kabul bombing when communities across Afghanistan are marking a day of special religious significance," the organisation tweeted, adding that the "UN family extends deepest condolences to the many families affected."
Both the Taliban and ISIS have carried out extensive attacks against Afghan government, military and civilian targets this year.
The US is increasingly eager to end the 17-year-old conflict it launched when it invaded Afghanistan following the September 11 Pentagon and World Trade Centre attacks in 2001. To that end it has began talks with the Taliban – which controls or influences nearly half of Afghanistan's districts, more than at any point since it was ousted from power by the US invasion.
The most recent meetings were held in Qatar, but talks are preliminary and an agreement is far from being reached.
Recent suicide attacks in Kabul have targetted schools, government buildings and community centres.
It has been a particularly bloody fighting season for Afghan forces, who have suffered an unprecedented number of casualties.
The Nato mission in Kabul said this summer's casualty rate has been the worst ever for Afghan forces. Afghan officials refuse to publish death tolls but US Secretary of Defence James Mattis said recently that 1000 Afghan soldiers and police died in August and September.
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