A bomb-rigged vehicle exploded outside Afghanistan's Supreme Court in a suicide attack today, killing at least 21 people and wounding about 40 others, police and witnesses said. All the dead were believed to be civilians, mainly court workers.

The attacker detonated the vehicle in the parking lot of the court compound, located several blocks from the US Embassy in the Afghan capital Kabul just as employees were leaving. The dead included nine women and a child, according to the Ministry of Public Health.

No group immediately claimed responsibility for the attack, but Taliban insurgents have conducted numerous attacks against judges, courthouses and legal officials. In 2013, a Taliban bomber drove a car full of explosives into a bus carrying Supreme Court workers, after which high blast walls were built around the complex. In 2015, a bomber killed five federal prosecutors in the capital.

President Ashraf Ghani condemned the attack as a "crime against humanity and an unforgivable act". An official of the Nato military mission in Afghanistan, Italian Army Lieutenant General Rosario Castellano, said "anyone who seeks to destabilise the pillars of a functioning government are enemies of Afghanistan".

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The daytime attack came less than a month after insurgent bombers killed more than 30 people and wounded more than 70 in twin blasts near the national Parliament building, and carried out bombings in two provincial capitals the same day that killed more than 20 people.

The latest assaults also followed alarming international reports on the Afghan conflict.

One tally indicated that a record number of civilians were killed and wounded in fighting last year as the Taliban continued to regain territory, leaving only about 57 per cent of the country under firm government control.

The UN report on casualties said almost 3500 Afghan civilians died and about 7900 were injured last year.