France told Russia on Monday that Nato would be prepared to defend the sovereignty of Ukraine, near where the alliance says Moscow has been staging a build-up troops.
The waring came as western leaders continued attempts to resolve a migrants crisis on the eastern borders of the EU.
The EU agreed to step up sanctions against Belarus over the thousands of migrants stranded in freezing forests on its borders with the EU. Belarus, a close Russian ally, said assertions it had engineered 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, French President Emmanuel Macron 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 Mr Macron told reporters.
Kremlin spokesman Dmitry Peskov had earlier dismissed as “wrong” a US 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 into the bloc to avenge earlier sanctions over a crackdown on protests last year against veteran President Alexander Lukashenko's contested re-election.
Belarus and Russia have both repeatedly denied any role.
Nato Secretary General Jens Stoltenberg said earlier on Monday that the allaince did not want to speculate on Russia's intentions on Ukraine.
“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,” he said.
Latvia said on Monday it had sent 3,000 troops for a previously unannounced military exercise near the border. Latvia, Lithuania and Poland make up the eastern flank of the EU and Nato, while Ukraine is not a member of either western group.
Several hundred migrants, some throwing stones, made an attempt on Monday to cross the border near the Polish village of Starzyna but were forced back, Polish police said on Twitter.
German Chancellor Angela Merkel and Mr Lukashenko discussed humanitarian aid for refugees and migrants by telephone, a German government representative said.
The talks are the first known contact between the Belarusian president and a western leader since last year's election in Belarus triggered mass protests by demonstrators accusing Mr Lukashenko of electoral fraud, a charge he denies.
Mrs Merkel and Mr Lukashenko agreed to continue their exchange, the official said, but gave no sign of a breakthrough. Mrs Merkel has spoken twice to Mr Putin in recent days.
Airlines
Middle East travel agencies working together with operators in Belarus provided tourist visas to thousands of people in recent months, a Reuters investigation showed.
The EU's executive, the European Commission, said it was looking into whether other airlines should face sanctions after the bloc banned Belarus' state-owned carrier Belavia from its skies and airports. Ireland said EU aircraft leasing contracts with Belavia would also end.
Mr Lukashenko said Belarus was trying to persuade migrants to go home but that none of them wanted to return. Minsk would retaliate against any new EU sanctions, he said.
The EU has been stepping up sanctions on Belarus for months. Curbs already in place include blacklisting of Mr Lukashenko, his son and 165 other Belarusian officials, as well as restrictions on trade in potash, an important export.
In Washington, the White House said it was in close contact with EU allies to hold the government of Belarus accountable.
The Kremlin, which has sent strategic bombers to patrol over Belarus, said Mr Putin spoke to Mr Lukashenko on Sunday and Moscow had no plans to reroute gas flows away from Belarus despite Minsk threatening to cut transit to Europe through the Yamal pipeline.
At least eight people have died along the 200km land border between Poland and Belarus, including from cold and exhaustion. The sparsely populated area of lakes, swamps and forests is becoming even more hostile to people trying to keep warm around bonfires through the cold November nights.
The EU's foreign policy chief, Josep Borrell, urged Warsaw to allow humanitarian aid to be distributed on the frontier, where Poland has stationed about 20,000 police, border guards and soldiers.
