New Delhi, India (CNN) Pakistan has promised retaliation "at the time and place of its choosing" after India conducted airstrikes on an alleged terrorist training camp inside Pakistan territory, in a significant escalation of hostilities between the nuclear armed powers.

Pakistan's Prime Minister, Imran Khan, directed the country's armed forces to remain prepared for all eventualities in response to the strikes, the first such incursion by Indian Air Force planes across the border between the two countries since the India-Pakistan war of 1971.

"India has committed uncalled for aggression to which Pakistan shall respond at the time and place of its choosing," Khan's office said in a statement on Tuesday.

Earlier on Tuesday, Indian Foreign Secretary Vijay Gokhale told reporters in New Delhi that "a very large number" of militants were "eliminated" in the operation, which he said came in response to "credible intelligence" about potential terrorist attacks.

Pakistan disputed India's claim that it targeted a terrorist camp and denied that numerous militants had been killed, calling it "a self serving, reckless and fictitious claim" by the Indian government.

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