A suicide bomber struck an Afghan military bus in Kabul on Saturday, officials said, causing casualties as insurgents step up strikes in the final weeks of NATO's combat mission in the country.

The Taliban claimed responsibility for the blast, which left the bus badly damaged as wounded victims were rushed to hospital.

"A suicide attacker targeted an ANA (Afghan National Army) bus," defence ministry spokesman Zahir Azimi said via Twitter. "(It) caused casualties. Numbers will be released later."

A Kabul police spokesman said the suicide bomber had been on foot.

Military buses are regularly targeted by the Taliban.

On Thursday a suicide bomber killed six soldiers travelling on a bus in the Kabul suburbs.

Earlier Saturday, Taliban gunmen shot dead a senior Supreme Court official as he left his home.

A spike of attacks in recent weeks have heightened concern about stability as NATO readies to end its combat mission on December 31 after a war that has failed to thwart the Taliban insurgency.

Other recent targets have included the French cultural centre, a female Afghan member of parliament, a British embassy convoy and a three members of South African family killed when their compound was attacked.