Britain's foreign secretary, meanwhile, said Islamist militants pose a “mortal threat'' to Pakistan and urged the country's feuding president opposition leader to unite to defeat it.

The remarks were the most direct appeal yet by a Western power for the two men to resolve their differences – and a sign London is taking their rift and its implications seriously.

The attack Tuesday on the Sri Lankan team was dramatic reminder of the threat posed by militants to nuclear-armed Pakistan.

The team and its security detail were attacked by about a dozen assailants with rocket launchers, hand grenades and assault rifles. Six police and a driver were killed, while seven players and a coach were wounded, none seriously. All the attackers escaped.

Police have said they are questioning several people in connection with the attack.

On Friday, a senior police investigator said that officers had received information a banned Sunni militant group played role. He gave no more details, and spoke on condition of anonymity because of the sensitive nature of the issue.

Salahuddin Niazi, the officer in charge of the probe, refused to confirm or deny that, saying only police were “on the right track.''

The assault bore similarities to November's three-day terrorist rampage in the Indian financial capital of Mumbai. The Pakistani group Lashkar-e-Taiba has been blamed for that attack, in which 164 people were killed. Many analysts have suspected the group - or an offshoot - carried out Tuesday's ambush.

Criticism of security before and during the ambush is adding to political problems facing President Asif Ali Zardari.

Last month, the Supreme Court banned opposition leader Nawaz Sharif and his brother, Shahbaz Sharif, from contesting elections in a decision widely seen as being influenced by Zardari, who promptly dismissed the Shahbaz-led government in Pakistan's most populous province.

Mr. Sharif is pledging to join large anti-government rallies scheduled for later this month. Many analysts are predicting months of turmoil, uncertainty and likely street violence - a combination the West does not want a year into the term of the country's first democratically elected government in 10 years.

British Foreign Secretary David Miliband said it was “vital'' the two men “unite against the mortal threat that Pakistan faces, which is a threat from its internal enemies.''