Russia warned Ukraine against integration with NATO on Tuesday as its alleged troop withdrawal from Ukraine's eastern border came under doubt by the military alliance, which said it will look at all options to allay the concerns of eastern Europe's former communist states prompted by Russia's annexation of Crimea.

"Unfortunately, I cannot confirm that Russia is withdrawing its troops. This is not what we are seeing," Secretary General Anders Fogh Rasmussen told reporters in Brussels.

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 mechanized infantry, armored 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.

Ministers from the 28 alliance members are meeting in Belgium 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.

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.

At the same time, the Russian Foreign Ministry warned Kiev integration into NATO, and said that Ukraine's previous attempts to move closer to the West had strained ties with Russia and caused problems between Moscow and NATO.

"(Past attempts) led to a freezing of Russian-Ukrainian political contacts, a headache between NATO and Russia and ... to a division in Ukrainian society," the Russian Foreign Ministry said as NATO foreign ministers gathered for a meeting in Brussels.

It warned that future economic ties between Moscow and Kiev "will largely depend on the actions Ukraine takes in its foreign policy."

The statement underscored Russia's sensitivity to contacts between Moscow's former Cold War enemy NATO and Ukraine, with whom Russia's relations have deteriorated since the removal of Moscow-backed Viktor Yanukovich as president on Feb. 22.

Moscow sees Ukraine, a former Soviet republic, as part of its traditional sphere of influence and wants a neutrality clause included in Ukraine's constitution to prevent it joining NATO.

Ukraine pursued NATO membership during President Viktor Yushchenko's rule from 2005 until 2010. The Kremlin has sought, where possible, to keep buffer states between Russia and NATO.