"The main message: There will be more support and more advanced support, heavier weapons, and more modern weapons, because this is a fight for our values," Stoltenberg said, at the World Economic Forum in Davos.

The Norwegian head of the US-led military alliance said "there is an urgent need, time matters" on sending more weaponry to Kyiv as it seeks to push Moscow's forces back.

"It's very dangerous to underestimate Russia," Stoltenberg said.

"They have mobilised 200,000 more troops. President (Vladimir) Putin has demonstrated a will to just sacrifice thousands and thousands of young Russian soldiers."

Speaking alongside Stoltenberg, Polish President Andrzej Duda echoed the call for urgency.

"There is a lot of Russian soldiers in the front line and we are afraid that they are preparing themselves for a new offensive, in a few months probably," Duda said in English.

"So this is crucial to send now additional military support to Ukraine, especially more modern tanks, especially modern missiles to defend Ukraine."

NATO's 30 member countries and some international partners will meet Ukrainian envoys on Friday at the Ramstein base in Germany to pledge more arms to Kyiv.

The government in Berlin is under pressure from Ukraine and other Western allies to greenlight sending Leopard tanks to help fight Putin's troops.

Britain this week became the first NATO member to make a firm commitment to send the modern, Western-designed battle tanks that Kyiv has been demanding.