NATO allies and partners have provided Ukraine with 1,550 armoured vehicles and 230 tanks to form units and help it retake territory from Russian forces, NATO chief Jens Stoltenberg said Thursday.

The deliveries, since the start of the war in February last year, represent "more than 98 percent of the combat vehicles promised to Ukraine", Stoltenberg told a news conference.

"In total we have trained and equipped more than nine new Ukrainian armoured brigades. This will put Ukraine in a strong position to continue to retake occupied territory," he said.

NATO member countries have also provided anti-aircraft systems and artillery while Poland and the Czech Republic have given Soviet-built MiG-29 aircraft.

Thousands of Ukrainian soldiers have been trained on weapons used by NATO.

Stoltenberg emphasised this "unprecedented military support to Ukraine" but cautioned that "we should never underestimate Russia".

Moscow was mobilising more ground forces and was "willing to send in thousands of troops with very high casualty rates," he said.

In the face of what appears will be a prolonged conflict, NATO countries "must stay the course and continue to provide Ukraine with what it needs to prevail", the alliance chief said.

Stoltenberg said that a NATO summit in July in Lithuania would set out plans for a "multi-year programme of support" for Ukraine.

He also welcomed a conversation Wednesday between Ukrainian President Volodymyr Zelensky and Chinese President Xi Jinping -- their first call since Russia's invasion of Ukraine.

Beijing says it is neutral in the Ukraine conflict and Xi has never condemned the Russian invasion.

The Chinese leader, who made a friendly visit to Moscow last month, has come under increased pressure from Western nations to step in and mediate.

"Maybe this war will end at the negotiating table," Stoltenberg said.

But he stressed that "it is for Ukraine to decide what are the conditions for talks and what format any talks should have".

In any case, Stoltenberg said, "any possibility for meaningful negotiations requires that Ukraine has the necessary military strength to send a very clear message to President (Vladimir) Putin that he will not win on the battlefield".

Last week, representatives of NATO members gathered in Ramstein in Germany for talks hosted by the United States to review the defence systems and supplies Ukraine says it needs.

Zelensky has urged Western allies to send modern fighter aircraft and long-range missiles to help repel Russian troops, but NATO countries have so far stopped short of supplying Western-made jets.

The Kremlin has sought to portray the involvement of NATO countries in bolstering Ukraine's defences as the alliance being directly involved in the conflict, something the alliance denies.