There are warnings the migrant crisis on the European Union's eastern borders could escalate into a military confrontation, as Belarusian President Alexander Lukashenko threatens to cut gas supply into the EU.
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.
The Belarus defence ministry earlier said that in response to a build-up of Polish military forces near the border it would be obliged it to take "appropriate response measures", both independently and together with its strategic ally, Russia.
While not an EU member, Ukraine, wary of becoming another flashpoint, 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 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.
Mr Lukashenko threatened to retaliate, including by shutting down the transit of Russian natural gas via Belarus.
There was no immediate response from Russia, its ally and financial backer.
Bombers, tensions
Russian President Vladimir Putin has said Moscow will try to help Europe weather an energy crunch and is hoping German authorities will soon certify the Nord Stream 2 pipeline that will carry more Russian gas to Europe's biggest economy.
Moscow reacted angrily in the past when Ukraine, another transit country, disrupted gas supplies to the West, but Mr Lukashenko has pushed back against its wishes at times while accepting loans and subsidised energy.
Minsk said two Russian strategic bombers were patrolling Belarusian airspace for a second day in a show of support.
In comments carried by local media, Mr Lukashenko also said there were attempts to transfer weapons to the migrants, but neither provided any evidence nor said who was behind what he called a provocation.
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 said Mr Putin held a second phone call in two days with German Chancellor Angela Merkel telling her the EU should talk to Belarus about the crisis.
The Kremlin earlier rejected as "crazy" a suggestion in a media report that Russia's flag carrier Aeroflot could be targeted with sanctions.
The EU has not said which airlines will be included, but Turkey also responded angrily to a report that its flag carrier Turkish Airlines might be affected.
"We reject efforts to portray Turkey, which is not a party to this issue, as part of the problem," the Turkish foreign ministry said in a statement.
Freezing weather
Trapped between two borders, the migrants have endured freezing weather in makeshift camps. Poland has reported at least seven migrant deaths in the months-long crisis and other migrants have expressed fear they would die.
None of around 150 migrants gathered near the town of Bialowieza managed to breach the border, with 468 attempted crossings on Wednesday, according to the Polish border guard service.
Neighbouring Lithuania, which like Poland has imposed a state of emergency on the border, also reported new crossing attempts.
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.
The Iraqi embassy in Moscow on Thursday said it was ready to help evacuate any Iraqi nationals who wanted to return home from Belarus, inviting them to contact it by WhatsApp or email.
EU foreign ministers may approve more Belarus sanctions on Monday that could include individuals and companies, according to one diplomat. 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," German Finance Minister Olaf Scholz, who is set to become the country's next chancellor, said.
Mr Lukashenko and Russia have said the EU was not living up to its humanitarian values by preventing migrants from crossing.
Large groups fleeing conflicts and poverty in the Middle East and elsewhere started flying to Minsk this spring with the help of Belarusian travel agencies.
Many have travelled to the border with Poland, Lithuania or Latvia and tried to cross into the EU, sometimes using wire cutters they say were given to them by Belarusian border guards.
