Security has been stepped up at Northern Ireland's main airports in the wake of an attack on Glasgow airport.
Police are on alert at Belfast International Airport
A burning car crashed into the front of the main terminal building at Glasgow on Saturday afternoon.
Belfast International Airport said two extra police vehicles had been drafted in and patrolling by airport police had been intensified.
The George Best Belfast City Airport said security had been increased as a precaution. That airport remains open.
Police are now holding four people following the Glasgow attack.
It is being linked to the discovery of two car bombs in London's West End.
A burning car crashed into the airport at Glasgow
The UK has moved to its highest level of terror alert - critical.
Two men were arrested, one who is in hospital with severe burns, after Saturday's attack, and two arrests were made on the M6 in Cheshire overnight.
Eyewitnesses have described a Jeep Cherokee being driven at speed towards the building with flames coming out from underneath.
They have also described seeing two men, one of whom was on fire, who had been in the car.