NATO said Wednesday it had seen no sign of Russian troops withdrawing from the border with Ukraine, despite President Vladimir Putin's claim they have been pulled back.

"On the claims that Russia is pulling back troops, I am not able to confirm that information -- we haven't seen any signs that Russia is withdrawing its troops," NATO Secretary General Anders Fogh Rasmussen said in Warsaw.

Putin had said earlier that in response to Western concerns of a possible Ukraine invasion, Moscow had pulled troops back.

"We were told constantly about concerns over our troops near the Ukrainian border. We have pulled them back," Putin said.

"Today they are not at the Ukrainian border but in places of regular exercises, at training grounds," he said after meeting Swiss president and current OSCE chief Didier Burkhalter.

US and NATO officials estimate Russia massed 40,000 troops on the border with Ukraine as the Kiev government battles pro-Kremlin militias.

Rasmussen on Wednesday also said NATO "will not hesitate to take further steps if necessary" to beef up defences on its eastern fringe.

"Those steps might include enhanced exercises, revised defence plans and a proper deployment," he said following talks with Poland's foreign and defence ministers.

As tensions with Russia over the Ukraine crisis flare, NATO military chief US General Philip Breedlove said Tuesday the alliance's build-up in Eastern Europe could become permanent.

The 28 members of the North Atlantic Treaty Organization have responded to Russia's intervention in Ukraine by stepping up defences in Eastern Europe, sending warships, fighter jets and troops to the region.

The troop surge is scheduled to end on December 31.