Pakistan has warned it will "avenge" those killed by Indian air strikes that New Delhi said were in response to an attack in Kashmir, signalling an imminent escalation in the worst violence in decades between the nuclear-armed neighbours.

At least 43 deaths have been reported so far, with Islamabad saying 31 civilians were killed by the Indian strikes and firing along the border, and New Delhi adding at least 12 dead from Pakistani shelling.

"We make this pledge, that we will avenge each drop of the blood of these martyrs," Pakistani Prime Minister Shehbaz Sharif said in an address to the nation late Wednesday.

India's army said it destroyed nine "terrorist camps" in Pakistan in the early hours of Wednesday, two weeks after New Delhi blamed Islamabad for backing an attack on tourists in the Indian-administered side of disputed Kashmir -- a charge Pakistan denies.

Pakistan military spokesman Ahmed Sharif Chaudhry said five Indian jets had been downed across the border.

An Indian senior security source, who asked not to be named, said three of its fighter jets had crashed on home territory.

The two sides have exchanged heavy artillery fire along the Line of Control that divides Kashmir,........

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