EU blames Putin for rising tension on Belarus and Ukraine

He also corroborated reports Russia had massed forces near Ukraine's eastern border, saying it appeared ready to launch an invasion "in just a few days' time" if it chose to.

Asked if Russia had coordinated the Belarus skirmishes and its renewed Ukraine threat, Borrell said: "I don't know the secrets of Lukashenko and Putin's talks".

But Polish foreign minister Zbigniew Rau went further, saying the Belarus and Ukraine emergencies were part of the same "hybrid aggression" against the West.

"Lukashenko is acting on behalf of his principal [Putin]," Rau said.

France also went further, even promising to protect Ukraine.

"Our willingness to defend Ukraine's sovereignty and territorial integrity was reiterated by the president," an aide to French leader Emmanuel Macron said in Paris after Macron phoned Putin on Monday.

The Kremlin, which has denied any wrongdoing, complained of Western "provocations", referring to Black Sea military drills, in its readout of the Macron call.

But "we have to be clear-eyed ... and what we see is a significant, large Russian military buildup," Nato secretary general Jens Stoltenberg also said after meeting Ukrainian foreign minister Dmytro Kuleba in Brussels.

The situation was "dangerous", Stoltenberg said.

Belarusian soldiers have been tearing down Polish border fences and forcing migrants to break through after trafficking tens of thousands of people from the Middle East.

Belarus has also menaced Lithuania and Latvia, which sent 3,500 soldiers to its border on Monday.

Meanwhile, Russia has some 114,000 forces deployed in and around Ukraine, Ukrainian military intelligence chief Kyrylo Budanov told the Financial Times.

And "information from our [military intelligence] coincides with information from partner countries about the high probability of destabilisation ... this winter," Ukrainian deputy defence minister Hanna Mailar said.

For his part, the French foreign minister, Jean-Yves Le Drian, highlighted that Russia was harming EU interests further afield as well.

Russia is planning to send mercenaries from the Kremlin-linked Wagner private military group to Mali, a region in which French forces historically held sway.

"There's a common will to decide a legal framework for sanctions that will be put on the Wagner Group," Le Drian said in Brussels on Monday.

EU ministers agreed a fifth package of sanctions on Belarus, targeting officials and entities behind the migrant operation, as well as others guilty of brutalising pro-democracy activists.

They would blacklist foreign airlines and travel agencies that did not stop flying people in, Borrell warned.

Lukashenko's use of vulnerable people who had been "weaponised, instrumentalised, cheated" was "illegal and inhuman", Borrell noted.

He had held a rare phone call with Belarusian foreign minister Vladimir Makei to ask him to stop, but Makei "denied any responsibility for those people [migrants] being there", Borrell said.

German chancellor Angela Merkel also phoned Lukashenko in an even rarer event.

They discussed humanitarian aid for refugees and agreed to maintain contact, a readout said.

And for his part, Poland's Rau felt "very satisfied" with the level of solidarity on the border crisis.

"I'm satisfied we achieved complete unity on the [Belarus] sanctions," he told Brussels media, while remarking on "the scale of natural support" among his fellow EU ministers for Poland at Monday's talks.