A suicide bomber has killed 12 people in Pakistan's capital in a sharp escalation of militant violence that the defence minister says had pushed the country into a "state of war".

Pakistani government ministers accused neighbouring Afghanistan of complicity in the bloodshed - an accusation officials in Kabul denied - and vowed retaliation if Afghan authorities failed to rein in the militants they say were responsible.

"We are in a state of war," Defence Minister Khawaja Muhammad Asif said after the attack, the first strike on civilians in Islamabad in a decade.

"Bringing this war to Islamabad is a message from Kabul, to which Pakistan has the full power to respond."

Pakistan is locked in confrontation with Afghanistan and India, fighting a four-day war with India in May and then last m

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