Pakistani investigators are sifting through evidence after a suicide bombing that killed 15 people.

The blast targeted a crowd of policemen guarding a protest to mark the anniversary of last year's raid on the radical Red Mosque.

13 policemen and two civilians died in the attack in the capital Islamabad.

There's been no immediate claim of responsibility but officials say the mosque's former students and Taliban militants are among the possible suspects.

A security official says the police, who were deployed at a rally by Islamist hardliners to mark the first anniversary of the deadly siege and storming of the Red Mosque, were targetted.

The Interior Ministry Secretary, Kamal Shah, earlier said it was premature to blame anyone.

"It will take a little time," he said.

The blast happened several hundred metres from the Red Mosque, after a tightly guarded meeting of Islamists at the mosque had ended and police were returning to their stations.

More than 100 people were killed when commandos stormed the Red Mosque complex on July 10 last year, after a week-long siege that began when gunmen from the mosque clashed with police outside.

The United States denounced the attack, calling it a 'needless act of violence' and vowing to help Pakistan fight their 'common enemy'.

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