Airlines operating flights from East Asia to destinations in Europe are having to reroute their planes away from Pakistan and northern India.
The airspace is closed because of escalating tension between the two countries, following the shooting down of two Indian military jets.
Flights via Pakistan have been cancelled and other flights rerouted.
Thai Airways has taken the more drastic step of suspending all its flights destined for Europe.
With flight space south of Pakistan becoming crowded, the Bangkok-based airline has not been able to establish alternative routes for its flights.
"By closing the airspace, every flight from Thailand to Europe has been affected. For flights that are going to depart this evening, we will call an urgent meeting to consider the impact of such events," said Thai Airways president Sumeth Damrongchaitham.
Singapore Airlines and British Airways are among the operators which have had to reroute flights. Singapore Airlines said longer flight routes would make refuelling necessary.
Alex Seftel, who works as a journalist, was en route from Bangkok to London on Wednesday on a flight with Taiwanese operator Eva Air. The flight was turned back over Calcutta in northern India.
"We were on the flight, a couple of hours in, and I noticed on the flight route map that it was going in the opposite direction," he said.
"There was a lot of circling around and we had very little information until we got into the airport."
Back in Bangkok, passengers waited several hours for an explanation before being transferred to a hotel for the night, with a new flight provisionally scheduled for early Thursday.
Some international flights have been rerouted through Mumbai on India's western coast.
Mark Martin, founder and chief executive at Martin Consulting India, said about 800 flights a day used the India-Pakistan air corridor, making it "very critical".
"You can't overfly China, so you have to overfly Pakistan and India and go to South East Asia and Australia. Most of the traffic destined for Bangkok and Singapore will have to fly over Iran and then possibly take a detour," he said.
The recent flare-up between Indian and Pakistan over the disputed region of Kashmir began when a suicide car bomb killed 40 Indian paramilitary police on 14 February. India retaliated with an airstrike on what it said was a militant training base on Tuesday.
Indian domestic airlines, including IndiGo, Go Air, Jet Airways and Vistara, cancelled services in northern India because of airport closures, although Indian airports later resumed operations.
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