A US-led group of some 50 countries held talks at NATO headquarters in Brussels with the focus on air defences after Russian President Vladimir Putin unleashed a barrage across Ukraine following a blast at a bridge to the annexed Crimea peninsula.

Ukrainian Defence Minister Oleksiy Reznikov said just three words when asked what he hoped for from the meeting: "Air defence systems."

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

"The systems will be provided, as fast as we can physically get them there," United States Defence Secretary Lloyd Austin said after the meeting, without giving details on any new pledges.

"We're going to provide systems that we have available ... We're also going to try to provide additional munitions to the existing systems that the Ukrainian forces are using."

A first Iris-T medium-range system has arrived in Ukraine after Germany decided to ship it before even giving it to its own troops.

The United States has also said it is looking to expedite the delivery of its NASAMS anti-missile and anti-drone system to Kyiv and a first batch of two is expected in the coming weeks.

Deliveries of a further six units could take far longer as they need to be manufactured and US sources said Washington is eyeing the possibility of trying to get Cold War-era Hawk systems to Ukraine in the meantime.

"There's other systems out there throughout the world that are available," US top general Mark Milley said.

"The task will be to bring those together, get them deployed."

- 'Pivotal moment' -

Ukraine's President Volodymyr Zelensky had called on G7 leaders on Tuesday to help establish an "air shield" over his country more than seven months into the war against Moscow.

Austin said that the resolve of Ukraine's allies to support Kyiv had been "heightened by the deliberate cruelty of Russia's new barrage against Ukraine's cities".

"Those assaults on targets with no military purpose again revealed the malice of Putin's war of choice," he said.

NATO defence ministers, who will meet on Thursday, are pushing for ways to bolster their overall weapons stockpiles as the war in Ukraine has depleted their shelves.

NATO members have supplied weaponry worth billions of dollars to help fight Russia's more than seven-month invasion of Ukraine and have vowed to keep supplies flowing as Kyiv pushes to recapture occupied territories.

"Allies have provided support to Ukraine by reducing NATO stocks, or ammunition, or weapons. This has been the right thing to do, but of course, we need to address how to refill those stocks," Stoltenberg said.

"I expect that the ministers will agree to review our guidelines for stocks and also to engage more with industry."

The NATO chief said the meeting in Brussels comes at a "pivotal moment" as Putin has followed up battlefield losses by annexing seized territory and issuing veiled nuclear threats.

Western powers say they have seen no change in Moscow's nuclear posture that would suggest it is getting ready to launch a strike. They have warned Moscow against deploying any small, tactical atomic bomb in Ukraine.

"There would be a sharp response -- almost certainly drawing a physical response from many allies, and potentially from NATO itself," a senior NATO official said.