By Saeed Shah and Asif Shahzad

ISLAMABAD (Reuters) -A suicide bomber killed 12 people in Pakistan's capital on Tuesday in a sharp escalation of militant violence that the defence minister said had pushed the country into a "state of war".

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

"We are in a state of war," said Defence Minister Khawaja Muhammad Asif 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 Kabul and New Delhi, fighting a four-day war with India in May and then last month carrying out airstrikes in Afghanistan, including Kabul, in response to what it said was the presence of Pakistani militants there. Subsequent skirmishes on the Pakistan-Afghan border were followed by unsuccessful peace talks.

No group claimed responsibility for Tuesday's attack, in which a suicide bomber blew himself up outside a busy lower court in Islamabad. It happened hours after militants stormed a school........

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