The night sky was lit up by a fire at Karachi International airport following a gun attack that killed five people and injured 10.
Smoke billowed from the airport in the aftermath of the attack.
The Pakistan army has now arrived at the scene.
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The threat level has been raised to critical by MI5's Counter-Terrorism Analysis Centre.
Prime Minister Theresa May announced the decision from Downing Street.
A man, apparently armed with a small axe and a large stick, was detained close to the event and police cleared the area as a precaution.