Ukraine's President Volodymyr Zelensky pleaded Thursday with NATO leaders to send weapons "without restrictions" as the alliance sought to bolster Kyiv and strengthen its eastern flank in the face of Russia's war.

The United States said allies were discussing sending anti-ship missiles to Ukraine as President Joe Biden sought to up the price for the Kremlin at a day-long string of summit in Brussels.

"To save people and our cities, Ukraine needs military assistance without restrictions," Zelensky told NATO leaders in a video address as the war entered its second month.

"The Alliance can still prevent the deaths of Ukrainians from Russian strikes, from Russian occupation, by giving us all the weapons we need."

NATO has rebuffed Ukraine's calls to intervene and impose a no-fly zone to stop Russia's ferocious bombardment -- and Kyiv is now focused on getting more advanced weaponry to defend itself.

"Vladimir Putin has already crossed the red line into barbarism," British Prime Minister Boris Johnson said.

"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 NATO's 30 nations 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."

NATO chief Jens Stoltenberg insisted the alliance was "determined to continue to impose costs on Russia to bring about the end of this brutal war".

But he ruled out NATO imposing a no-fly zone 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.

NATO leaders are vowing to bolster weapons deliveries to Ukraine and supply protection against chemical and nuclear threats from Russia
NATO leaders are vowing to bolster weapons deliveries to Ukraine and supply protection against chemical and nuclear threats from Russia POOL via AFP / Brendan SMIALOWSKI

The NATO chief 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 said 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 leaders also looked to put pressure on China to end its political support for Moscow, amid fears that Beijing could send military hardware to help the Kremlin.

"We need to continue to call on China not to support Russia in its aggression against Ukraine," a US official said. "We need China to call for a peaceful end of the conflict as a responsible member of the international community."