A powerful bomb attack on a police academy bus may be one of the deadliest such attacks since the fall of the Taliban, and it points to the risks faced by Afghanistan's police officers.

Gen. Rick Hillier, Canada's chief of defence staff, told reporters in Kandahar on Sunday that training for Afghan police has been a neglected area.

"So there is a window of perhaps more vulnerability with the police which the Taliban have seen. That's why they go after them. We've got to work with them to make sure that window is closed as rapidly as possible," he said.

At least 35 people were killed when the bomb detonated in Kabul, the nation's capital, on Sunday.

Purported Taliban spokesperson Qari Yousef Ahmadi said the strike was carried out by a suicide bomber. The Associated Press reported that the claim could not be verified.

Afghan authorities aren't sure whether a suicide bomber carried out the attack or whether the militants managed to smuggle a bomb aboard the bus or plant a roadside bomb.

In any event, the explosion was huge.

"Never in my life have I heard such a sound," Ali Jawad, a 48-year-old who was selling phone cards near the blast site, told The Canadian Press. "A big fireball followed. I saw blood and a decapitated man thrown out of the bus."

The explosion occurred at 8:10 a.m. local time.

Body parts were littered as far as 20 metres away from the wreckage of the bus, which was completely destroyed, and investigators were attempting to collect the bodies of the victims.

Twenty-two policemen were among the dead, said Ahmed Zia Aftali, the head of Kabul's military hospital. That infers that 13 civilians were among the victims. A doctor at a nearby hospital said at least 52 were wounded, with the majority of them in serious condition.

Nasir Ahmad was sitting at the back of the bus. "There were between 30 to 40 police instructors in the bus," the survivor told AP, speaking from a hospital bed where he was recovering from wounds to his face and hands.

CTV's Paul Workman said violence appears to be on the rise in Afghanistan. Sunday's attack is reminiscent of al Qaeda tactics in Iraq that have proven effective against U.S. troops and Iraqi police, he said.

"As I was told earlier this week by a senior Canadian commander here in Kandahar, there seems to have been an upsurge certainly in roadside bombings in southern Afghanistan, and this is the second suicide bombing or serious bombing we've seen in Kabul in the last two days," Workman told CTV Newsnet from Kandahar.

In addition to the police transport, a civilian bus was also damaged in the strike. The bus was driving just ahead of the police academy vehicle, and its position may have helped shield civilians, said a doctor at the scene.

Soft targets

Suffering from poor equipment and training, Afghanistan's police suffer from one of the highest casualty rates in the world. At least 307 have been killed this year alone, according to a count by The Associated Press.

Workman said a Canadian commander told him Afghan police remain a "soft target" for the Taliban.

"They don't have the kind of heavy duty armour, heavy weapons that the military does, both the Canadian and NATO military and the Afghan military, and that they seem to have become the new prime target of the Taliban."

By coincidence, a new training mission has begun for Afghan police officers. While Europeans are leading the work, Canadians are involved.

With a report from CTV's Paul Workman and files from The Canadian Press and The Associated Press