Belarus has said it will no longer stop migrants who reach its territory from crisis regions from continuing on to Poland and trying to cross into the European Union.
The EU has however accused Belarus of abetting migrants trying to reach the bloc's external borders in Poland, Latvia and Lithuania - even flying in asylum seekers - in retaliation for European sanctions on Belarus for political repression.
Poland is refusing to let the migrants onto its territory and has sent hundreds of reinforcements to the border in recent weeks as well as erecting a barbed wire fence in an effort to stop any attempts by migrants to break through.
The government has furthermore imposed a state of emergency on the border area which outlaws border crossings.
Polish officials said that some migrants had already tried to break through, with the Defence Ministry posting a video of the incident on its Twitter feed on Monday.
The Belarus Foreign Ministry on Tuesday warned Poland "not to use any provocations directed against the Republic of Belarus to justify possible illegal military actions against disadvantaged unarmed people".
Belarus security forces were also on their way to the border area where the migrants had gathered, the Polish Defence Ministry said, showing a video of a line of marching uniformed men in the woods close to the border.
Polish President Andrzej Duda accused Belarusian forces of blocking the migrants in the border zone so they had no option but to stay there.
Two UN bodies, the UNHCR refugee agency and the migration agency IOM, said they were alarmed at the situation and demanded humanitarian access to the region.
Neither NGOs nor journalists are being allowed access to the area.
"The instrumentalisation of migrants and refugees to achieve political ends is deplorable and must stop," they said.
"Taking advantage of the despair and vulnerability of migrants and refugees by offering them unrealistic and misleading promises is unacceptable and has severe human consequences," they added.
The EU said on Tuesday it would suspend a visa deal with Belarus to allow government officials to travel more easily into the bloc.
"It is unacceptable for Belarus to play with people's lives for political purposes," Slovenian Interior Minister Ales Hojs said in a statement sent out by the Council of the EU, which represents the 27 member states.
On Monday, Belarusian authorities said that many hundreds more migrants had arrived at the border near the town of Bruzgi.
Poland has put the total number now there at between 3000 and 4000.
The temperature in the region hovered around freezing overnight and many of the migrant groups had lit fires to keep themselves warm.
Many thousands have already crossed the border at some point this year - 8000 people arriving from Belarus had registered in Lithuania, Poland and Latvia, according to the European Commission.
Many others have made their way further west on to Germany.
The EU is currently planning fresh sanctions against Belarus, principally to target state airline Belavia for allegedly transporting migrants to Belarus.
Turkish Airlines on Tuesday denied reports that it was helping Lukashenko put pressure on the EU by flying migrants from Turkey into Belarus.
NATO Secretary General Jens Stoltenberg said he had spoken to Polish President Andrzej Duda about the situation.
"Belarus using migrants as a hybrid tactic is unacceptable," Stoltenberg tweeted, referring to the use of non-military and military tactics as a means of destabilising target countries.
"NATO stands in solidarity and all our allies in the region," he wrote.
Lukashenko's main backer is Russian President Vladimir Putin.
The two leaders spoke on Tuesday about the "harsh actions of the Polish side," according to Lukashenko's office.
The deployment of the Polish army to the border was of particular concern, Lukashenko said.
A Kremlin spokesman said that he hoped the situation did not escalate into a "security risk".
