''More than 35 are killed,'' said Ali Shah Paktiawal, chief of the Kabul police's criminal branch. ''Police officers are among the dead.'' An Interior Ministry official could not confirm the death toll, saying around 30 were dead or wounded.

The Interior Ministry said five of the wounded were foreigners, including two Japanese, a Korean and two Pakistanis. Earlier reports that foreigners had been killed proved false.

If the death toll is confirmed, this would be the deadliest suicide attack to hit the country since the ouster of Taliban from power.

The attack appears to signal an escalation of such strikes.

At least 14 people were killed in four other suicide bombings over the past three days. Responsibility for all five attacks was claimed by Taliban insurgents. The previous deadliest bombing in Kabul, in 2002, killed 26 people.

A police eyewitness at the scene, outside the Kabul police chief's headquarters, said he had seen the bomber leap on to the bus as it was moving slowly away, its door wide open.

''It was a very, very successful suicide attack,'' a Taliban commander, Mullah Hayatullah Khan, told Reuters by satellite phone. ''We have plans for more successful attacks in future.'' The Taliban and their al Qaeda allies have adopted the tactics of Iraq's bloody insurgency to try to dispel the notion that government and foreign security forces are in control of the country.

Eighteen bodies, mostly police officers, and 10 wounded had been taken to nearby Jamhuriat Hospital, a doctor there said.

The bomb exploded during the morning rush hour, at a time when buses are ferrying police officers to their beats. On Friday and Saturday there were four suicide attacks in the south, centre and north of the country, including a blast in Kabul yesterday.