Malaysia Airlines re-routed a Kuala Lumpur-to-London flight over Syrian airspace on Sunday after its usual route over Ukraine was closed, flight tracking data showed on Monday.
Flightradar24 posted a flight map on its Twitter account on Monday showing the change in the flight's route.
Flight tracking data showed this flight had previously crossed over eastern Ukraine.
After Malaysia Airlines' Flight MH17 was shot down last Thursday by a ground-to-air missile in eastern Ukraine, airlines began to avoid the airspace below which the Ukrainian military has been fighting with Russian-backed rebels.
On Friday and Saturday the Kuala Lumpur-London flight, MH4, had taken a different route over eastern Turkey, tracking data showed.
The change in route on Sunday highlights the challenge that airlines face in finding conflict-free passageways on the congested routes between Asia and Europe.
— Flightradar24 (@flightradar24) July 21, 2014
Hundreds of flights routinely crossed over Ukraine before Thursday's incident, and it is not unusual for international airlines to overfly war zones such as Syria or Afghanistan.
The US Federal Aviation Administration, whose regulations are among the world's strictest, "strongly discourages" US operators from flying to, from or over Syria, according to a May 2013 notice on its website.
A statement from Malaysia Airlines acknowledged that it was flying over Syrian airspace.
"MH004's flight plan is in accordance to International Civil Aviation Organisation's (ICAO) approved routes," the statement said.
"As per the notice to airmen (NOTAM) issued by the Syrian Civil Aviation Authority, the Syrian airspace was not subject to restrictions. At all times, MH004 was in airspace approved by ICAO."
Sydney Morning Herald/Reuters