Ministers from the 28 alliance members are meeting in Brussels for the first time since Russia's military occupation and annexation of Ukraine's Crimea region triggered the worst East-West crisis since the Cold War.
They will discuss ways to boost NATO's military presence in former communist central and eastern Europe to reassure allies rattled by Russia's moves.
"Unfortunately, I cannot confirm that Russia is withdrawing its troops. This is not what we are seeing," Secretary-General Anders Fogh Rasmussen told reporters at the start of the talks.
Diplomats said the ministers would consider options ranging from stepped-up military exercises and sending more forces to eastern member states to the permanent basing of alliance forces in those countries - a step Moscow would view as provocative.
Asked if NATO could station forces permanently in the small former Soviet Baltic states, Rasmussen said: "We are now considering all options to enhance our collective defence, including ... further development of our defence plans, enhanced exercises and also appropriate deployments."
Polish Foreign Minister Radoslaw Sikorski told reporters as he arrived that he would welcome "some more prominent NATO presence in Poland."
Some NATO members are cautious about taking steps that could aggravate the crisis, particularly after Moscow said on Monday it had pulled some troops back from near the Ukrainian border.
But a NATO military official, speaking on condition of anonymity, said Russia still had some 35,000-40,000 troops stationed near the border and that there was no sign of any significant reduction in their numbers.
The Russian forces included mechanised infantry, armoured units, special forces, logistics units and "fairly substantial numbers" of planes and helicopters, the official said.
There was also little evidence that the troops were there for training, the official said. There were some exercises but other units were moving to a location and staying put.
"It's an indication of troops given orders to deploy somewhere and awaiting further orders," he said.
The Russian forces did not pose a threat to NATO countries but could pose a threat to Ukraine, the official said.
The United States and its allies have made clear they have no military plans to defend Ukraine, which is not a NATO member.
Since the Crimea crisis erupted, the United States has increased the number of U.S. aircraft in regular NATO air patrols over the Baltic States and has beefed up a previously planned training exercise with the Polish air force.