Two employees of Afghanistan's human rights body were killed in a bomb attack in Kabul on Saturday, the agency said.

The Afghanistan Independent Human Rights Commission said the pair died when a homemade "sticky bomb" attached to their vehicle exploded in the morning.

Kabul police spokesman Ferdaws Faramurz confirmed the attack, which has not been claimed by any group.

The United Nations Assistance Mission in Afghanistan condemned the killings.

"There can be no justification for attacks against human rights defenders," it said on Twitter, calling for an immediate probe.

It comes less than a week after two prosecutors and three other employees from the attorney general's office were shot dead by gunmen on the outskirts of Kabul.

On May 30, a television journalist was killed when a minibus carrying employees of private television channel Khurshid TV was hit by a roadside bomb in the city. That attack was claimed by the extremist Islamic State group.

Violence had dropped across much of the country after the Taliban offered a brief ceasefire to mark the Eid al-Fitr festival last month, but officials say the insurgents have stepped up attacks in recent weeks.

Most attacks by the Taliban have targeted Afghan security forces, although there are regular police reports that civilians have been killed in roadside bomb blasts.

On Saturday, the National Security Council said 21 civilians were killed and 30 wounded in attacks over the past week across 14 provinces.

The Taliban and Afghan government are preparing to enter into much delayed peace talks aimed at ending the war in the country.

US finalizing plan to pull 4,000 troops: CNN

CNN on Saturday reported that the Trump administration is close to finalizing a decision to withdraw more than 4,000 troops from Afghanistan by the fall, citing two Trump administration officials.

Almost a week ago, Gen Kenneth McKenzie, head of the US Central Command, said the United States has reduced its troop level to 8,600 in Afghanistan, fulfilling the first phase of the planned withdrawal specified in the US-Taliban deal signed in Doha in February.

The new move would reduce the number of troops from 8,600 to 4,500 and would be the lowest number since the very earliest days of the war in Afghanistan, which began in 2001.

This comes a day after a meeting between US Defence Secretary Mark Esper and NATO Secretary General Jens Stoltenberg in Brussels. “We will also follow up on our discussion on Afghanistan. NATO will continue to adjust our presence in support of the peace process,” Stoltenberg said on Friday. “This will be done in close coordination with Allies and partners.”

The US has committed to withdrawing all of its forces from Afghanistan by next April under the agreement it signed with the Taliban.

It is unclear when the withdrawal of the 4,000 additional US troops would take place, CNN reports.