Afghanistan’s main intelligence agency says its officers have arrested at least 26 suspected members of the extremist group Islamic State (IS), accusing them of plotting attacks on members of the Shi'ite minority.

The militants, including an alleged IS leader, were detained in separate operations in Kabul, the National Directorate of Security (NDS) said on September 18.

A statement said they planned to target Shi’a during this week’s commemoration of Ashura, the holiest celebration in the Shi'ite religious calendar.

The IS affiliate in Afghanistan has stepped up attacks on minority Shi’a across Afghanistan.

The group claimed responsibility for a September 9 twin bomb attack at a wrestling club in a predominately Shi’ite neighborhood of Kabul that killed more than 20 people.

IS militants assert that Shi’a are apostates who deserve death.

Afghan forces have struggled to combat the Taliban and IS since the United States and NATO formally ended their combat mission in the country in 2014.

In the northern province of Balkh, a local official said on September 18 that nine members of the Afghan police force were killed when another officer shot them at a checkpoint.

Mohammadudin Khanjer, a police official in the district of Char Boldak, said one officer was wounded and three others were missing after the shooting late on September 17.

The attacker was a policeman from another checkpoint in the same district who fled the scene after seizing all weapons from the checkpoint, apparently to join the Taliban, Khanjer said.

The Taliban did not immediately comment on the attack but the militants are active in the area.

Insider attacks, in which members of Afghan security forces or assailants dressed in Afghan uniforms fire on coalition troops, have become less common in recent years but remain a persistent worry.

The U.S. military said one of its servicemen was killed and another was wounded in eastern Afghanistan on September 3 in an attack carried out by a member of the Afghan National Police.

It came nearly two months after a member of a U.S. Army training unit was shot dead by an Afghan soldier in the southern province of Oruzgan.

With reporting by AP and Khaama Press