Thousands of migrants were massing near the Belarus border on Monday, Polish authorities said, as European Union member states called for more sanctions against Minsk and security forces braced for more attempts to force through the frontier.
Warsaw has accused Belarus of trying to spark a major confrontation, with video clips showing hundreds of migrants walking towards the Polish border and some trying to breach the fence using spades and other implements.
"We expect that in the coming hours, attacks on our border will be renewed by groups of several hundred people," Pawel Soloch, the head of Poland's National Security Bureau, told reporters.
Polish government spokesman Piotr Muller said that there were currently 3,000-4,000 migrants near the border, and more than 10,000 others across Belarus ready to try and cross into Poland.
Warsaw said it had deployed additional soldiers, border guards and police while neighboring Lithuania said it might introduce a state of emergency on its border with Belarus.
Poland said it had withstood the first attempts on Monday by the migrants to force their way across the border.
The Polish Border Guard announced that as of 0600 GMT on Tuesday the crossing at Kuznica, near the site where migrants tried to force their way through, would be closed.
"We have three border crossings with Belarus," Deputy Interior Minister Maciej Wasik told private broadcaster Polsat News. "The closure of one of them can... be treated as a kind of economic sanction."
Wasik said there should be tougher EU sanctions against Minsk following the escalation of tensions on the border. The Baltic states Lithuania, Latvia and Estonia have issued similar calls.
European Commission President Ursula von der Leyen said she was calling for member states to approve extended sanctions against the Belarusian authorities.
The EU also wants to target airlines from third countries that help transport the migrants from the Middle East to Belarus.
The European Union accuses Minsk of encouraging migrants from the Middle East and Africa to cross into the EU via Belarus, as a form of hybrid warfare in revenge for Western sanctions on President Alexander Lukashenko's government over human rights abuses.
A video distributed by Polish authorities showed one man cutting part of a barbed-wire fence, another attacking the fence with a spade, while a Polish soldier sprayed an unidentified substance from a can.
In an earlier video, shared by the Belarusian blogging service NEXTA, migrants carrying rucksacks and wearing winter clothing were seen walking on the side of a highway. Other videos showed large groups of migrants sitting by the road and being escorted by armed men dressed in khaki.
Lithuania said it also was moving additional troops to the border to prepare for a possible surge in migrant crossings. Latvia said the situation was "alarming".
Lukashenko's government has repeatedly denied manufacturing a migrant crisis, blaming the West for the crossings and treatment of migrants.
The Belarusian state border committee confirmed on Monday that many refugees were moving towards the Polish border, but said Warsaw was taking an "inhumane attitude".
Poland has stationed more than 12,000 troops at the border, its defense minister said, while sharing aerial footage of migrants clustered on the Belarusian side.
"They throw tree trunks on the fence so as to reduce the height of this fence to breach it," said Katarzyna Zdanowicz, spokeswoman for Polish border guards in the area.
The EU, the United States and Britain imposed sanctions on Belarus after Lukashenko unleashed a violent crackdown on mass protests following a disputed election last year.
The US State Department on Monday said it was concerned with disturbing images and reports from the border, and called on the government of Belarus to immediately halt its campaign of orchestrating and coercing irregular migration flows across its borders.
The United Nations called the scenes at the border "deeply concerning".
Lukashenko has defied opposition calls to resign, buttressed by money and diplomatic support from traditional ally Russia.
Kremlin spokesman Dmitry Peskov on Monday defended Minsk's handling of the migrant issue, saying Belarus was taking all necessary measures to act legally.
Charities say the migrants face freezing weather conditions and a lack of food and medical attention.
Poland said seven migrants had been found dead on its side of the border, with reports of more deaths in Belarus.
Humanitarian groups accuse Poland's ruling nationalists of violating the international right to asylum by pushing migrants back into Belarus instead of accepting their applications for protection. Poland says its actions are legal.
Poland's Prime Minister Mateusz Morawiecki said on Facebook: "The Polish border is not just a line on a map. The border is sacred - Polish blood has been spilled for it!".