NATO member Romania announced Thursday that it would send a Patriot missile system to Ukraine, welcomed by Kyiv as a "crucial contribution" to help its fight against Russia's invasion.

While Kyiv is calling for more Patriot missile systems, most NATO countries have been reluctant to send them because they want to protect their own airspace.

"Considering the significant deterioration of the security situation in Ukraine... council members decided to donate a Patriot system to Ukraine in close coordination with allies," Romania's Supreme Council of National Defence said in a statement.

The donation was made "on the condition that our country continues negotiations with allies, in particular the US, with a view to obtaining a similar or equivalent system" to protect its own air space, it added.

The country, which borders Ukraine, also needed "a temporary solution to cover the operational vulnerability thus created", it added.

Ukraine President Volodymyr Zelensky welcomed Romania's "principled support".

"This crucial contribution will bolster our air shield and help us better protect our people and critical infrastructure from Russian air terror," he posted on X, formerly Twitter.

Germany recently announced it would transfer a third Patriot air-defence system to Ukraine, while the United States is expected to send a second battery to Kyiv, according to US media reports.

Romanian President Klaus Iohannis indicated an openness to help Kyiv with a Patriot system at the beginning of May, after meeting US President Joe Biden in Washington.

But he warned it was "unacceptable to leave Romania without air defences".

Romania signed a $4-billion deal for seven Patriot batteries with the US in 2017, the biggest defence acquisition in its history.

Two of the four systems it has received so far are fully operational.

Romania has been providing military help to Kyiv in the war with Russia, but has refused to reveal the scale of the support, citing security concerns.

It has also pledged to train Ukrainian pilots to fly F-16 jets in a regional hub inaugurated in November 2023, although the timeline for that programme remains unclear.