A suicide bomber attacked a Shiite Muslim shrine in central Kabul on Tuesday where a crowd of hundreds had gathered for the festival of Ashura, killing up to 54 and 150 injured people in what appeared to be an unprecedented sectarian attack.

Mohammad Zahir, head of Kabul's Criminal Investigation Department, said he had counted up to 20 bodies at a Kabul hospital, and expected the toll to rise.

Afghanistan has a history of tension and violence between Sunnis and the Shiite minority, but since the fall of the Taliban the country had been spared the large-scale sectarian attacks that have troubled neighbouring Pakistan.

The noon bomb in a riverside shrine, in the heart of old Kabul, appears to set a grim new precedent.

Shortly after, a bicycle bomb exploded near the main mosque in the northern city of Mazar-i-Sharif, killing four people and injuring 17 others.

The city's streets were filled with people celebrating Ashura, but it was not immediately clear if that attack was also targetting Shiite worshippers.

Taliban spokesmen could not immediately be reached for comment on either attack.

The Shiite Muslim festival of Ashura marks the martyrdom of the Prophet Mohammad's grandson Hussein in the battle of Karbala in Iraq in the year 680 BC.

Ashura is the biggest event in the Shiite Muslim calendar, when large processions are vulnerable to militant attacks, including suicide bombings. Pakistan has deployed tens of thousands of paramilitary soldiers and police during Ashura.

Blood has spilled between Pakistan's majority Sunni and minority Shiite militants for decades.

Sectarian strife has intensified since Sunni militants deepened ties with Al Qaeda and Pakistani Taliban insurgents after Pakistan joined the US-led campaign against militancy after the 11 September attacks.