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 E.U. 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 E.U. 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 E.U., the authorities in Warsaw said.
Download the NBC News app for breaking news and politics
The E.U. 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 seven migrant deaths in the months-long crisis and other migrants have expressed fear they would die.
Neighbouring E.U. 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.