At least 20 people were killed killed, 50 others wounded when a suicide bomber blew himself up inside a Shiite mosque in Kabul on Friday, the latest violence to strike the Afghan capital.

Kabul police spokesman Abdul Basir Mujahid told AFP a suicide bomber "detonated himself inside the mosque", adding that there were casualties but he could not confirm how many.

The interior ministry confirmed a "terrorist attack" had taken place while residents told AFP they could hear gunshots.

The attack occurred at a time when many Muslims would be attending Friday prayers, and raises fears of sectarian violence in the capital.

It also comes days after U.S. President Donald Trump cleared the way for thousands more American troops to be deployed in the war-torn country.

Najib Danish, a deputy spokesman for the interior ministry, told TOLOnews that initial information suggested a suicide bomber detonated himself, while two or three other terrorists had entered the mosque and were exchanging fire with police.

"The bombers are running short of bullet rounds and they are using knives to stab worshippers," an eyewitness told AFP.

The assault underscores spiraling insecurity in Afghanistan as a resurgent Taliban steps up offensives across the country, while the Daesh terrorist group, known for carrying out sectarian attacks, expands its Afghan footprint.

The Taliban have denied their involvement in this sixth incident of its kind this year.

According to the UN Assistance Mission in Afghanistan, four of the previous attacks occurred in western Herat province bordering Iran while the other two in Kabul. Daesh had claimed responsibility for two of these attacks.

Earlier this month, up to 31 people were killed following a suicide attack and subsequent gun battle at a Shia mosque in Herat.

Last year, the UN mission recorded four separate attacks against Shia mosques and religious gatherings, two of which were claimed by Daesh.