Sophisticated NATO surveillance planes have flown their first mission in support of the US-led coalition battling the Islamic State group, alliance head Jens Stoltenberg said Tuesday.

NATO leaders agreed in July to commit the AWACS planes after Washington had pressed hard for the alliance to make a concrete gesture to help the fight against IS.

"NATO itself is now offering direct support with our AWACS surveillance aircraft," Stoltenberg told a press briefing in Brussels, announcing that the first flight took place on October 20.

NATO was "committed to sustaining the coalition's momentum so that IS can be defeated once and for all," he said at alliance headquarters ahead of a two-day defence ministers meeting in Brussels.

All 28 NATO members also belong to the anti-IS alliance but some were reluctant to see the coalition directly involved in the Syrian conflict, so the aircraft were limited to flying in international airspace or over NATO member Turkey, from where they can look deep into Syria and Iraq.

Stoltenberg said that while he could not comment on their exact role, the aircraft "will not be part of combat operations."

The planes are one of the few concrete assets that NATO has, with most of its military hardware belonging to individual member states.

AWACS have powerful radars that allow them to monitor airspace for hundreds of kilometres (miles) around and they can also serve as command posts to coordinate bombing raids and other operations.