Polish Prime Minister Mateusz Morawiecki on Sunday called the migrant crisis on the Belarusian-Polish border, the EU's eastern frontier, the "greatest attempt to destabilize Europe" since the Cold War.
The premier issued his strong remarks as he prepared to meet with EU leaders at a time when Warsaw is facing not only a border crisis but heightened tensions with Brussels over allegations it is breaching its commitment to the bloc's democratic principles.
The West accuses Belarus of artificially creating the crisis by bringing in would-be migrants — mostly from the Middle East — and taking them to the border with promises of an easy crossing into the European Union.
Belarus has denied the claim, instead criticizing the EU for not taking in the migrants.
Caught in the middle, migrants often report being forced to cross the border by Belarusian officials, then being pushed back into Belarusian territory by Polish authorities.
Belarusian President Alexander "Lukashenko launched a hybrid war against the EU. This is [the] greatest attempt to destabilize Europe in 30 years," Morawiecki said on Twitter.
"Poland will not yield to blackmail and will do everything to defend the EU's borders."
He linked to a video statement in which he cautioned that "today the target is Poland, but tomorrow it will be Germany, Belgium, France or Spain."
He also claimed that Lukashenko had the "back-room support of Vladimir Putin," the Russian president and an ally of the Belarusian regime.
Lukashenko told the BBC earlier that it was "absolutely possible" his forces had helped people cross into the EU but denied orchestrating the operation.
Brussels and NATO have previously also described the migrant crisis as a "hybrid tactic."
Morawiecki is visiting the Baltic states — two of which also share a border with Belarus — on Sunday to discuss the conflict and has announced he will visit other EU capitals this week.
Some observers believe Poland is using its rhetoric on the border issue to try to distract from controversial reforms that the EU believes limit the independence of the judiciary.
The European Commission wrote to Poland on Friday to launch a process that could lead to it being deprived of funds over threats to the EU legal order.
"While the problem on Poland's border is serious and requires Western solidarity — for example by sanctioning Belarus — Morawiecki blows it out of proportion to divert attention from Poland's violation of the rule of law," political expert Marcin Zaborowski told AFP.
The policy director at the Globsec think tank argued that the Belarus action "pales in comparison with the war in Ukraine, cyber attack in Estonia in 2007 and Russian support for far-right extremism in Europe."
On Sunday, Poland's border guards reported new attempted crossings, including by a "very aggressive group of around 100" migrants.
Poland's Defense Minister Mariusz Blaszczak said Saturday that Belarus has now changed tactics by directing smaller groups of migrants to multiple points along the border.
He added that he expected the border showdown to last months.
The migrants have spent thousands of dollars to fly into Belarus on tourist visas, determined to reach the European Union.
Many are desperately fleeing war and poverty in the Middle East.
Once at the border, they are faced with squalid, freezing conditions.
Polish media say at least 11 migrants have died since the crisis began over the summer.
A Yemeni migrant who died in Poland in September will be laid to rest on Sunday in the eastern village of Bohoniki, with his brother in attendance.
Yemen's foreign ministry said he "died on the border between Poland and Belarus as a result of the severe drop in temperatures."
On Saturday, the Belarusian Health Ministry said that a World Health Organization (WHO) mission had arrived in Belarus to help organize medical support for the migrants.