NEW DELHI (Reuters) – Indian airlines cancelled service to least six cities in northern India on Wednesday and several airports were closed as tensions with neighbouring Pakistan escalated.

IndiGo, India’s biggest airline by market share, low-cost rival GoAir and full-service carriers Jet Airways and Vistara, a joint venture between Singapore Airlines and Tata Sons said flights to several airports were on hold or temporarily suspended.

The airlines said on Twitter that flights to cities including Amritsar, Chandigarh, Dehradun, Jammu, Leh and Srinagar have been affected due to airspace restrictions or airport closures.

India on Wednesday shut several airports in the northern part of the country after Pakistani warplanes crossed a disputed border into the Indian side of Kashmir.

The escalation by Islamabad followed an Indian airstrike on a militant camp inside Pakistan a day earlier, raising the risk of conflict between the two countries.

Airports in the cities of Amritsar, Pathankot, Jammu, Leh and Chandigarh have been shut, sources told Reuters.

The main airport in Srinagar, the summer capital of Jammu and Kashmir, was closed for three hours, police in the city said.

At Jammu airport in Kashmir, hundreds of passengers crowded airline counters to cancel their tickets after the airport was closed.