The death toll in the deadly suicide bombing near the Lal Masjid in Islamabad rose to 20 with a policeman succumbing to his injuries on Monday even as Prime Minister Yousuf Raza Gilani said the attack was aimed at destabilising Pakistan.

A constable of the Special Branch, who was injured in Sunday's suicide attack on a police contingent, died in a Islamabad hospital on Monday morning. Officials said 18 of the people killed were policemen. Over 40 people were injured in the attack.

The government has announced a reward of Rs 5 million for information on the perpetrators of the suicide bombing, which occurred shortly after the end of a meeting organised by hardline religious groups to mark the first anniversary of the storming of the Lal Masjid by the army last year.

Speaking to reporters before leaving for Kuala Lumpur to attend the D-8 conference, Prime Minister Gilani said the incident had brought a bad name to Pakistan. "It's a cowardly act by those who want to destabilise the country and malign Islam," he said.

Rehman Malik, who functions as the interior minister, said a reward of Rs 5 million will be given to anyone who helps identify the suicide bomber and other perpetrators of the attack.

The government has formed a joint investigation team comprising officials of the Inter Services Intelligence and Federal Investigation Agency.

The bomber's torso had been found at the site of the blast and a sketch of his face would soon be released, Malik told reporters after visiting victims of the attack in a hospital here late on Sunday night.

Malik also told Geo News channel that witnesses had said that the attack was carried out by a boy aged about 15 or 16 years who was clad in a white salwar-kameez. He had earlier said the attacker was in his thirties.

Pakistani officials said five kilograms of explosives were used in the suicide jacket worn by the bomber. More pieces of flesh and bones were collected by investigatorson Monday morning from the site of the blast and rooftops of nearby buildings.

Body parts of the bomber were hurled up to 50 yards away by the blast.

A handful of investigators scoured a grassy area near the site of the blast, looking for clues. Many people drove to the area on Monday morning to have a look at the site of the attack. The area had been closed to vehicular traffic for most of Sunday.

No group has claimed responsibility for the attack, though analysts said it was probably carried out to avenge the military siege of the Lal Masjid last year that left over 100 people dead.

In recent days, the Pakistani Taliban and other militant groups have warned they will retaliate against an operation launched by security forces in the Khyber tribal region.

Asked if the government's efforts to restore peace in the tribal areas would be affected by the attack, Prime Minister Gilani said it was too early to say anything in this regard. "I can't jump to a conclusion straight away," he said.

"Let (the law enforcing agencies) do the investigation. After the investigation, let them bring to the nation the real situation," he added.

Gilani also visited the Pakistan Institute of Medical Sciences and inquired about those injured in the attack. He told reporters at the hospital that an inquiry had been ordered and the perpetrators of the attack would not escape the law.

The funeral prayers of nine policemen killed in the suicide blast were offered during early on Monday morning at the Police Lines in Islamabad. The coffins containing their bodies were handed over to their families after the prayers.