Countries bordering Belarus, on Thursday, warned the migrant crisis on the European Union's eastern borders could escalate into a military confrontation, while Ukraine said it would deploy thousands more troops to reinforce its frontier.
Lithuania, Estonia and Latvia said Belarus posed serious threats to European security by deliberately escalating its "hybrid attack", using migrants to retaliate for EU sanctions.
"This increases the possibility of provocations and serious incidents that could also spill over into the military domain," a joint statement by the countries' defence ministers said.
While not an EU member, Ukraine is wary of becoming another flashpoint in the escalating migrant crisis. Kyiv announced drills and the deployment of 8,500 additional troops and police officers to the country's long northern border with Belarus.
Migrants stranded inside Belarus threw rocks and branches at Polish border guards and used logs to try to break down a razor wire fence overnight, in new attempts to force their way into the EU, the authorities in Warsaw said.
The EU says Minsk is encouraging thousands of migrants fleeing war-torn parts of the world to try to cross its borders and may impose new sanctions on Belarus and airlines ferrying the migrants as soon as Monday.
Belarusian President, Alexander Lukashenko, threatened to retaliate, including by shutting down the transit of Russian natural gas via Belarus, although there was no immediate response from Russia, its close ally and financial backer.
Russian President, Vladimir Putin, has said Moscow will try to help Europe weather an energy crunch and is hoping that German authorities will soon certify the Nord Stream 2 pipeline that will carry more Russian gas to Germany.
Moscow reacted angrily in the past when Ukraine, another transit country, disrupted supplies of gas to the West and Lukashenko has proved a difficult partner, pushing back against its wishes at times while accepting loans and subsidised energy.
Russia dispatched two strategic bombers to patrol Belarusian airspace on Wednesday in a show of support for its ally. Belarus said Russian planes carried out drills for a second day on Thursday.
"Yes, these are bombers capable of carrying nuclear weapons," Lukashenko said. "But we have no other option. We must see what they are doing there beyond the borders."
He also said there were attempts to transfer weapons to the migrants, in what he described as a provocation in comments carried by Belarus state media. He did not provide any evidence and it was not clear who he was accusing of doing this.
The Kremlin said Russia had nothing to do with tensions on the border and suggested the presence of heavily armed people on both sides was a source of concern. It also rejected as "crazy" a suggestion in a media report that Russia's flag carrier, Aeroflot, could be targeted with retaliatory sanctions.
The EU has not said which airlines will be included, but Turkey also responded angrily to the sanctions plan.
"We reject efforts to portray Turkey, which is not a party to this issue, as part of the problem," Turkish foreign ministry said in a statement, adding Ankara viewed the targeting of its flag-carrier, Turkish Airlines, over the matter as "intentional".
Trapped between two borders, the migrants have endured freezing weather in makeshift camps. Poland has reported at least even migrant deaths in the months-long crisis and other migrants have expressed fear that they would die.
None of around 150 migrants gathered near the town of Bialowieza managed to breach the border, a spokesperson for the Polish border guard service told Reuters, saying there were 468 attempted crossings on Wednesday.
Neighbouring EU state, Lithuania, which like Poland has imposed a state of emergency on the border, also reported new attempts to breach the frontier.
It said it had asked the United Nations to discuss creating a "humanitarian corridor" from the border zone to help the migrants return to their home countries, saying social media accounts showed some people trapped there wanted to go back.
Poland's deputy Foreign Minister, Pawel Jablonski, said the crisis was "the worst threat Poland has faced in the last thirty years", telling Italian daily, La Stampa, Warsaw expected an escalation in the coming days.
EU foreign ministers may approve a fifth Belarus sanctions package on Monday, which could include individuals and companies, a diplomat said on Thursday. The bloc's executive commission said airlines that bring migrants would be on the list and two diplomats said the main airport in Belarus was also being considered.
The EU accuses Lukashenko of manufacturing the crisis in revenge for earlier sanctions in response to a violent crackdown on mass street protests against his rule in 2020. Germany said he must be countered with all strength.
"Lukashenko is making an inhumane power play with people," said German Finance Minister, Olaf Scholz, who is set to become the country's next chancellor.
Lukashenko and Russia have said the EU was not living up to its humanitarian values by preventing migrants from crossing.
Large groups of people fleeing conflicts and poverty in the Middle East and elsewhere started flying to Minsk this spring with the help of Belarusian travel agencies. They then travel to the border with Poland, Lithuania or Latvia and try to cross into the EU, sometimes with wire cutters they say were given to them by Belarusian border guards.
EU Commission Vice President, Margaritis Schinas, was in the United Arab Emirates on Thursday as part of a trip to countries in the region whose airlines are operating flights to Belarus, diplomats and officials said.