NATO Secretary General Jens Stoltenberg expects a further intensification of the fighting and humanitarian crisis in Ukraine, he told the German newspaper Welt am Sonntag.
He said Ukrainians were resisting Russian forces with courage and determination, but the coming days could get worse.
Stoltenberg also dismissed recent "absurd claims" by Moscow that the US was secretly operating laboratories in Ukraine to make chemical and biological weapons.
"Now that these false allegations have been made, we must remain vigilant, because it is possible that Russia itself could be conducting missions with chemical weapons under this tissue of lie," he said.
He also rejected calls for NATO to impose and enforce a no-fly zone over Ukraine. "That could mean a direct confrontation and escalation. We need to end this war, not let it expand," he added.
As a message to Russian President Vladimir Putin, he said: "End this war, withdraw all your forces back and commit to diplomacy."