NATO chief Jens Stoltenberg on Thursday said the alliance remained ready to talk to Russia and seek common ground.
However, he said members feared Moscow was seeking a "pretext for an armed attack on Ukraine."
"There is still no clarity, no certainty about the Russian intentions," Stoltenberg told reporters.
The comments came at the end of talks over 2 days as NATO defense ministers met in Brussels amid persistent fears of a potential Russian invasion of Ukraine. They were joined by the defense ministers of Ukraine and Georgia.
The NATO head said the situation still remained unpredictable.
"We don't know what will happen. What we do know is that Russia has amassed the biggest force we've seen in Europe in decades in and around Ukraine," Stoltenberg said.
What have NATO members said about the Ukraine crisis?
Host of the meeting Stoltenberg on Wednesday dismissed suggestions that the threat on Ukraine's border had diminished amid Russia's claims of a withdrawal.
He maintained that there was still a strong threat of an attack, with an estimated 150,000-plus Russian troops surrounding Ukraine on three sides. Moscow claims its troops are there for exercises, and that some were being pulled back because the drills have been completed.
Stoltenberg said the alliance was looking to strengthen its eastern flank, warning that the threat from Russia had become the "new normal in Europe."
In a joint statement, the NATO allies called for Russia to reverse its troop build-up and engage in further talks.
"We urge Russia, in the strongest possible terms, to choose the path of diplomacy, and to immediately reverse its build-up and withdraw its forces from Ukraine in accordance with its international obligations and commitments. We remain committed to our dual-track approach to Russia: strong deterrence and defence, combined with openness to dialogue."
Some allies have already rushed to bolster defenses with thousands of troops and hardware amid the fears about Moscow's intentions.
The United States says it is temporarily deploying some 4,700 additional soldiers to EU and NATO member Poland in response to Russia's troop build-up near Ukraine.
