Ukrainian President Volodymyr Zelenskyy has appealed to NATO leaders to increase military support for his country against Russian attacks.
NATO leaders have pushed to raise the cost for Russian President Vladimir Putin over his country's attacks in Ukraine by bolstering weapons supplies to Kiev 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.
"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 conflict 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 attacks against its pro-Western neighbour.
"Putin cannot win this war," she said. "We have to stop the war criminal."
Ukrainian President Volodymyr Zelenskyy, whose country is not in NATO, appealed 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," Zelenskyy 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."
'Costs on Russia'
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 Kiev 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.
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".