A huge explosion has ripped through a crowded mosque in the Afghan capital Kabul, killing 21 people, police say.

Another 33 people were injured, Kabul police spokesman Khalid Zadran said.

Wednesday's blast occurred during evening prayers. The mosque's imam Amir Muhammad Kabuli is reported to be among the dead.

It is unclear who was behind the attack, a week after Islamic State (IS) militants killed a pro-Taliban cleric in a suicide bomb blast, also in Kabul.

Security forces have now sealed off the blast scene in Kabul.

Witnesses described hearing a powerful explosion which shattered windows in nearby buildings.

"I saw that so many people were killed, even people were thrown out of the windows of the mosque," one eyewitness told Reuters news agency.

Stefano Sozza, the head of the medical charity Emergency, which runs the city's main hospital, told the BBC his group had already treated 35 people, including children.

"Our doctors... operated [on] the patients in need of surgical assistance all night long. The injuries were mainly related to the blast, so there were shells inside the bodies and burning injuries all over the bodies of the victims."

He said he believed the blast was inside the mosque. "There was probably someone entering during the prayer time, so it was crowded at that moment and then [the attacker] activated this body burning improvised explosive device.

"So that all the persons that were close to the attacker were basically dead and the ones all around received several injuries," Mr Sozza added.

News agency AP quoted Taliban spokesman Zabihullah Mujahid as saying that the "perpetrators of such crimes will soon be brought to justice and will be punished".

Senior Taliban officials are meeting more than 2,000 tribal and religious elders on Thursday in Afghanistan's second city Kandahar, the movement's main power base.

A spokesman quoted by AFP news agency said "important decisions" would be taken at the major gathering.

The Kabul bombing comes days after the assassination of prominent cleric and Taliban supporter Rahimullah Haqqani in a suicide bombing.

The Islamic State group said it carried out the attack on Mr Haqqani, one of the highest profile figures to be killed since the Taliban returned to power a year ago.