(CNN) The European Union has said it will slap new sanctions on Belarus targeting "everyone involved" in facilitating a migrant crisis on the bloc's eastern frontier, as military moves and accusations continue to test a fragile political order in the region.
NATO Secretary-General Jens Stoltenberg on Monday warned Moscow against "potential aggressive actions" amid a large concentration of Russian troops on Ukraine's borders, deepening concerns over the potential for a wider geopolitical crisis.
"We have to be clear-eyed, we need to be realistic about the challenges we face. And what we see is a significant, large Russian military build-up," Stoltenberg told a news conference with Ukraine's Foreign Minister Dmytro Kuleba in Brussels.
Though he said did not want to speculate about Russia's intentions, he added: "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." Ukraine is not a member of NATO or the EU.
Secretary of State Antony Blinken said last week that Russia's recent movements were reminiscent of its invasion of Crimea in 2014.
The words of caution from NATO come days after Russia and Belarus flexed their military might near Poland in joint paratrooper drills.
Russia, Belarus' largest political and economic partner, continues to defend Minsk's handling of the border crisis while also denying any involvement in it.
On Monday, the EU ramped up pressure on all parties involved in the border crisis. The bloc's foreign affairs chief Josep Borrell announced in a news conference in Brussels that EU foreign ministers have agreed to impose new sanctions on "people, airlines, travel agencies and everyone involved in this illegal push of migrants against our borders."
The full list of people and entities would be "confirmed in the coming days," he added. This will be the fifth round of sanctions slapped on Belarus by the EU following a disputed Presidential election and crackdown on dissidents.
Borrell said earlier on Monday that he had spoken to the Belarusian foreign minister over the weekend, telling him "the situation on the border was completely unacceptable and that humanitarian help was needed."
Thousands of people are stuck in freezing conditions near the Kuźnica-Bruzgi checkpoint, trying to cross into Poland. Polish border guards used loudspeakers on Monday to warn people there that force may be used against them if they don't follow orders. The authorities also deployed a helicopter and a water cannon into the area, although the water cannon has not been used. A column of police vehicles has been seen approaching the border.
Poland, Lithuania and Latvia are considering triggering NATO's Article 4 over the crisis. Polish Prime Minister Mateusz Morawiecki told the Polish Press Agency on Sunday that his government is in talks with Latvia and Lithuania on whether to trigger the article, which calls for consultation when "the territorial integrity, political independence or security of any of the parties is threatened."
Katarzyna Zdanowicz, a spokesperson for the Polish Border Guard, told CNN on Monday morning that around 4,000 people were camped out along the border and that the situation was "very tense and very dangerous."
Zdanowicz said there had been "forced mass attempts to cross the border" in the Kuznica area by a group of 60 people on Sunday night. Those who tried to cross behaved "aggressively," according to the border guard service. They threw stones and branches at Polish border police, she said, adding that weapons were being "pointed towards our servicemen" and a "flare gun" was fired towards them.
More people were heading to the area on Monday. A CNN team witnessed thousands of people in the Bruzgi migrant camp in Belarus pick up their bags and start moving towards the Polish border on Monday morning.
A rumor started circulating in the camp that the Polish government might open the border and allow a humanitarian corridor through to Germany. Poland has adamantly denied this and people amassed in the area have received text messages from Polish authorities saying the information was a "total lie and nonsense."
The SMS message, also received by members of the CNN team in the area, reads in part: "Poland won't let migrants pass to Germany. It will protect its border. Don't get fooled, don't try to take any action."
Belarus' State Border Committee said the people moving towards the border have "purely peaceful intentions," according to Belarus state media BelTA.
"The refugees gathered warm clothes, tents, sleeping bags, hoping for a positive outcome of the situation. By noon, they self-organized into a large column and began to move towards the Bruzgi checkpoint," Anton Bychkovsky, the official representative of the border committee, was quoted as saying.