The blast occurred in the Serahi Alauddin area of the city, private Afghan channel TOLO news reported citing an eyewitness.
Emergency, a non-governmental organization that offers free medical and surgical treatment to Afghan civilians, said in a tweet that 20 wounded people had been treated at its hospital following the explosion.
At least two UN staff members and their families were inside the Khalifa Sahib Mosque at the time of the attack, the UN deputy spokesman for the secretary general said Friday.
Farhan Haq said he condemned the attack in the “strongest terms,” calling it “the latest in a series of indiscriminate assaults on civilian targets.”
In a statement posted on Twitter Friday, Taliban spokesperson Zabihullah Mujahid also said he condemned the attack, which took place days before the end of Ramadan.
“We strongly condemn today’s blast at a mosque in Kabul’s 6th District. Such brutal killings during this fast are the work of circles that have nothing to do with Islam or the country,” Mujahid tweeted on Friday.
“The Islamic Emirate extends its deepest sympathies to the families of the victims. The perpetrators of these crimes will soon be found and punished, God willing,” he added.
Weeks of deadly attacks
A series of deadly blasts, mainly targeting the Hazara Shia community, has rocked several Afghan cities over the past two weeks.
Last week, 33 people were killed and dozens were injured in a blast at a mosque in Afghanistan’s Kunduz province during Friday prayers, according to Taliban spokesman Mujahid.
That blast followed other attacks in previous days – one in the city of Mazar-i-Sharif in northern Afghanistan and another in Kunduz, along with multiple explosions earlier in the week at a high school and educational center in western Kabul.
Most of the recent attacks were claimed by ISIS affiliate group and Taliban rival ISIS-Khorsan (ISIS-K). Friday’s explosion has yet to be claimed by any group.
CNN’s Mostafa Salem, Nicola Careem and Hannah Ritchie contributed to this report.