MOSCOW, Nov 15 (Reuters) - France told Russia on Monday NATO would be prepared to defend the sovereignty of Ukraine, near where NATO says Moscow has been staging a troop buildup, while Western leaders sought to tackle a migrants crisis on the eastern borders of the European Union.

The European Union agreed to step up sanctions against Belarus over thousands of migrants stranded in freezing forests on its borders with the EU. Belarus, a close Russian ally, said assertions it had fuelled the crisis were "absurd".

Speaking by telephone to Russian President Vladimir Putin as part of a flurry of conversations between Western leaders and Russia, Belarus and Ukraine, the French leader spoke of his strong concern over the situation on Ukraine's borders.

"Our willingness to defend Ukraine's sovereignty and territorial integrity was reiterated by the president," an adviser to Macron told reporters of the conversation Macron initiated.

Kremlin spokesman Dmitry Peskov earlier dismissed as "wrong" a U.S. State Department statement that the Belarus border crisis was meant to distract attention from increased Russian military activity close to Ukraine, another former Soviet republic.

The EU is seeking to stop what it says is a policy by Belarus to push migrants towards it to avenge earlier sanctions over a crackdown on protests last year against veteran leader Alexander Lukashenko's contested re-election.

Belarus and Russia have both repeatedly denied any role.

NATO Secretary-General Jens Stoltenberg said earlier on Monday NATO did not want to speculate on Russia's intentions on Ukraine, while adding: "We see an unusual concentration of troops, and we know that Russia has been willing to use these types of military capabilities before to conduct aggressive actions against Ukraine."