At least 20 people were killed on Tuesday in a bomb blast outside the Supreme Court in the centre of Afghanistan's capital, Kabul, government officials said, in what appeared to be the latest in a series of attacks on the judiciary.

The Ministry of Public Health said at least 20 people were killed and 38 injured people were taken to city hospitals.

There was no immediate claim of responsibility for the attack, in which police said an apparent suicide bomber targeted Supreme Court employees leaving their offices at the end of the working day.

"When I heard a bang I rushed toward the Supreme Court's parking lot to find my brother who works there," said a witness, Dad Khuda, adding he had found his brother alive.

"Unfortunately, several people were killed and wounded."

Reuters witnesses at the scene reported blood stains on the street and numerous ambulances leaving the area.