NATO allies rejected Ukraine's demand for no-fly zones on Friday, saying they were increasing support but that stepping in directly would lead to a broader, even more brutal European war.
Ukraine, a former Soviet republic that wants to join the European Union and Western military alliance NATO, is not currently a member of either.
Support so far has come mainly in the form of far-reaching sanctions on Russia, with EU members on Friday saying more financial punishment was yet to come.
"We are not part of this conflict," NATO Secretary-General Jens Stoltenberg told a news conference.
"We have a responsibility as NATO allies to prevent this war from escalating beyond Ukraine because that would be even more dangerous, more devastating and would cause even more human suffering."
Ukrainian President Volodymyr Zelenskiy has called for Western powers to enforce a no-fly zone since Moscow's invasion started nine days ago, with Russia shelling cities and bringing fighting to Europe's largest nuclear plant.
In a video message earlier on Friday, Ukrainian Foreign Minister Dmytro Kuleba said NATO foreign ministers should not allow Putin to "turn Ukraine into Syria," in reference to the devastating Russia-backed campaign against rebel cities in that country.
"Act now before it's too late," he said in the message posted on Twitter.
Stoltenberg said NATO understood Ukraine's desperation, calling Russia's war in Ukraine "horrific" and warning the worst was yet to come, with Russia rolling out more heavy arms.
"But we also believe that if we did that (a no-fly zone) we would end up with something that could lead to a full-fledged war in Europe" involving many more countries, he said.
NATO members have been sending arms to Ukraine, but stopped short of military action.
Russia and NATO-members United States, Britain and France are all nuclear powers.
The only way for NATO to implement a no-fly zone would be to send NATO planes to shoot down Russian ones, Stoltenberg said, adding that the risk of escalation would be too big.
"Allies agree we should not have NATO planes operating over Ukrainian airspace or NATO troops operating in Ukrainian territory," he said.
US Secretary of State Antony Blinken said the alliance would defend "every inch" of NATO territory from attack. "Ours is a defensive alliance. We seek no conflict. But if conflict comes to us, we are ready," Blinken said.
Instead of a military presence in Ukraine, European Union countries - most of them also NATO members - said they were eyeing more economic sanctions to add to coordinated restrictions that have already targeted Russia's financial system and elites.
It was not immediately clear, however, when the 27-nation EU would be able to agree on exact measures given member states' divisions on doing business with Moscow and some countries' heavy reliance on Russian energy supplies.