(AFP) - NATO leaders pushed Thursday to raise the cost for Russian President Vladimir Putin over his invasion of Ukraine by bolstering weapons supplies to Kyiv and strengthening the alliance's eastern flank.

US President Joe Biden is looking to boost unity and ramp up sanctions on Moscow at a day-long string of summits in Brussels as the West responds to the Kremlin's bid to upend the post-Cold War balance.

"Vladimir Putin has already crossed the red line into barbarism," said British Prime Minister Boris Johnson as NATO leaders arrived for a meeting and the war marked one month.

"The harder our sanctions, the tougher our economic vice around the Putin regime, the more we can do to help the Ukrainians, I think the faster that this thing can be over," he said.

Estonian Prime Minister Kaja Kallas called on the 30-nation alliance to "double our efforts" to check the Kremlin's aggression against its pro-Western neighbour.

"Putin cannot win this war," she said. "We have to stop the war criminal."

Ukrainian President Volodymyr Zelensky, whose country is not in NATO, was set to appeal for more advanced weaponry and greater intervention in a video address to leaders as he seeks to cajole the West into a tougher response.

"We are waiting for meaningful steps. From NATO, the EU and the G7," Zelensky said ahead of the day of summits of all three organisations in Brussels.

"At these three summits we will see: Who is a friend, who is a partner, and who betrayed us for money. Life can be defended only when united."

NATO leaders are vowing to bolster weapons deliveries to Ukraine and supply protection against chemical and nuclear threats from Russia.

"We are determined to continue to impose costs on Russia to bring about the end of this brutal war," NATO chief Jens Stoltenberg said.

But the alliance has rebuffed pleas from Kyiv to impose a no-fly zone to help halt Russia's onslaught for fear of getting dragged into a "full-fledged" conflict with Moscow.

"We have a responsibility to ensure that this conflict does not escalate beyond Ukraine that will cause even more suffering, even more death, even more destruction," Stoltenberg said.

- 'Big mistake' -

He accused Putin of making a "big mistake" by attacking Ukraine and underestimating the strength of the Kyiv's resistance as its forces have stalled Moscow's advance.

Stoltenberg said the leaders of the US-led military alliance would "address the need for a reset of our deterrence and defence in the longer term", starting with agreeing new deployments to eastern members Romania, Hungary, Slovakia and Bulgaria.

NATO has already rushed tens of thousands of troops to its eastern flank in the wake of Russia's invasion to counter the threat of any spillover from the conflict into alliance countries.

Biden warned before heading to Europe of a "real threat" that the Kremlin could use chemical weapons in Ukraine.

Stoltenberg told journalists that "any use of chemical weapons would fundamentally change the nature of the conflict".

"It will be a blatant violation of international law, and it will have widespread and severe consequences."

Leaders refused to give details on how NATO would react if Moscow unleashed chemical weapons against Ukraine.

"I personally don't believe that Russian military itself will use chemical or biological weapons purposely for tactical achievements because this would be... very unwise and a shot in their own knee," said Slovenian premier Janez Jansa.

NATO allies are also to put pressure on China to drop what they see as its "political backing" for Russia amid fears that Beijing could start supplying military hardware to Moscow.

"I think the messages to China have been very, very clear," said Belgian Prime Minister Alexander De Croo -- "Please keep distance and up to now China has done so."

© Agence France-Presse