Ukraine's international supporters have held talks at the NATO headquarters in Brussels with the focus on air defences after Moscow unleashed a blitz across Ukraine following a blast at a bridge to the annexed Crimea peninsula.

Stoltenberg said the older Hawk launchers from Madrid would complement more modern systems being supplied by Germany, France and the United States.

"We have seen that when we mobilise, when we call on NATO allies to do more, they're actually doing more, and that's making a huge difference," Stoltenberg said after the NATO defence ministers meet.

US Defense Secretary Lloyd Austin thanked Spain for its "very, very rapid response" to Ukraine's latest request for more air defences.

"We would encourage the rest of our allies to dig deep and provide additional capability as well," he said.

There has not yet been any formal public announcement from the Spanish authorities.

Western allies are struggling to work out how to supply more advanced systems to Ukraine as diplomats admit they have precious few to spare.

American Hawk surface-to-air systems originally came into service during the Cold War and are no longer in use by the US military after being superseded by more modern Patriot units.

Spain has refurbished some of its Hawk systems in recent years to extend their shelf life.