Poland, the EU and the US have accused Belarus leader Alexander Lukashenko of exploiting migrants after a large group gathered at the border on Monday attempting to cross into Poland.
There was strong international condemnation of Belarus on Monday as the European Union, the United States and NATO pointed fingers at the Lukashenko government after a sudden surge in the number of migrants massing at the Polish border.
Poland accused Belarus of a "deliberate escalation of tension" on its side of the border between the two countries, after several hundred migrants gathered at the frontier trying to reach the European Union.
The European Commission accused the Belarusian leader of continuing "to use people as pawns" in his campaign against the bloc, saying he must "stop putting people's lives at risk".
Commission President Ursula von der Leyen called for extended sanctions, and possible measures against third-country airlines involved in human trafficking. Belarus has been accused of flying people from Middle Eastern countries and dumping them at EU borders.
A statement on the Polish government website published on Monday evening said "large groups of migrants" were massed at the border, under the "full control" of the Belarusian army and authorities.
"A coordinated attempt at a mass entry into the territory of the Republic of Poland of migrants used by Belarus for a hybrid attack against Poland has just begun," it said.
The statement said the latest influx was a "form of revenge" by Belarus' leader Alexander Lukashenko against Poland, Lithuania and Latvia.
"By creating an artificial migration route and cynically exploiting migrants, Lukashenka is trying to destabilize Poland, Lithuania and Latvia, and to force the European Union to lift the sanctions imposed on the Minsk regime."
Poland's interior ministry said it had rebuffed an attempt at illegal entry on Monday, saying the situation was under control.
Video posted to social media by Polish authorities showed migrants in tents on the border overnight, with police stating the night passed "relatively calmly".
The US State Department described the scenes at the border as "troubling" and called on Belarus to stop its "manipulation" of the situation.
NATO called it "unacceptable" and accused Lukashenko of "using" migrants as a "hybrid tactic".
Earlier, Warsaw reinforced security at the border, sending additional police. Polish defence minister Mariusz Błaszczak said more than 12,000 soldiers were on duty at the frontier with Belarus and that they were prepared to defend it.
Interior minister Mariusz Kaminski said that a "tough border defence" was the country's main priority.
"We have been monitoring the situation in Kuźnica for several days and we are prepared for any scenario," said Kaminski in a Twitter post.
"We increased the number of Border Guard officers, policemen and soldiers," he added.
The defence ministry posted a video showing a large group of migrants on the other side of a fence separating the two countries.
EU governments have also accused Belarusian President Alexander Lukashenko of deliberately supporting illegal migrants in response to EU sanctions against him.
Von der Leyen said two top EU officials — EU Commission vice president Margaritis Schinas and EU foreign affairs chief Josep Borrell — will travel in the coming days to the main countries of origin for the migrants to “ensure that they act to prevent their own nationals from falling into the trap set by the Belarusian authorities.”
European Commission spokesperson Adalbert Jahnz said earlier that the sudden influx was "a continuation of the desperate attempt by the Lukashenko regime to use people as pawns to destabilise the European Union and of course the values that we stand for and we have repeatedly firmly rejected attempts to instrumentalise people for political purposes".
Belarus' state border committee countered in a statement that the migrants were exercising "their right to apply for refugee status in the EU".
"All of these people, including women and children, do not pose a security threat and do not behave aggressively," the committee said.
Human rights groups have criticised EU governments for pushbacks of migrants on the border with Belarus, where many remain stranded, while calling on countries not to weaponise migrants.
In a tweet on Monday, the UNHCR wrote: "Images from Belarus-Poland border are deeply concerning. We have repeatedly said using refugees & migrants to achieve political ends is unacceptable & must stop. It’s time to act now – we call on Belarus to avoid putting lives at risk."
The UN refugee agency has implored Belarus and Poland to abide by legal obligations to provide asylum. They said there are reports of at least 10 migrants who have died at the border.
"People must be able to exercise their rights where they are, be it in Belarus or in Poland or other EU States where they may be located. This must include the possibility to seek asylum, access to legal aid, information and appropriate accommodation," said Pascale Moreau, UNHCR’s Regional Director for Europe, in a statement issued at the end of last month.
The US chargé d'affaires in Poland, Bix Aliu, tweeted: "Lukashenka's regime is putting the lives and health of migrants at risk, using them to escalate the border crisis and provoke Poland. Hostile actions by Belarus are exacerbating the situation on the border with the EU and NATO dangerously and must end immediately."
Last week, Poland summoned Belarus' chargé d'affaires over "unidentified uniformed individuals armed with long guns" on Polish territory, stating Belarus had escalated the situation.
The country's senate recently approved a plan to build a wall on its border with Belarus.
The European Commission said it was in touch with Polish authorities but had not had a specific request for support at the border from one of the EU's agencies.
Spokesperson Adalbert Jahnz said the Commission continued to encourage Poland to "avail itself of the support opportunities that exist at the European level" such as Frontex.
Some EU governments as well as MEPs have called for the EU to fund the construction of border walls to keep migrants from entering. But human rights groups oppose the idea.
Last month, Gerald Knaus, chairman of the European Stability Initiative (ESI), a European think tank specialising in asylum policy, warned the EU against entering "a contest of brutality" with Lukashenko.
He called on Brussels to find partner countries to work together to resist the "blackmail" coming from Minsk.
"The European Union should help these countries massively, and in return these countries could help the European Union. This is about protecting human lives, protecting human dignity but also not giving in to a dictator," he told Euronews.