Poland has signed an agreement with the UK for British military engineers to help install a solid wall on its eastern border with Belarus, to be completed by mid-2022.
Pressure began to ease at a border crossing on Thursday as Belarusian soldiers accompanied people to a nearby government-run warehouse.
Poland said it would maintain 15,000 soldiers in border regions, and border police said they had recorded 500 attempted border crossings from Wednesday to Thursday.
Polish prime minister Mateusz Morawiecki warned on Thursday that it was unclear what would happen next in a crisis that was becoming "more and more dangerous".
Faced this year with a growing wave of migration via Belarus, which Warsaw and its western allies say was being masterminded by the Minsk government, Poland began installing razor-wire fencing on almost half of its 400km border with Belarus.
On Thursday, Polish defence minister Mariusz Blaszczak said he had struck an agreement with his UK counterpart, Ben Wallace, to install a solid barrier, equipped with electronic surveillance equipment and topped with barbed wire.
“Our border is and will be tight; we will ensure Poland’s security ... with the support of British troops,” said Mr Blaszczak in a joint press conference with Mr Wallace in northeast Poland. “A company of British soldiers will be deployed to serve alongside Polish soldiers as soon as possible, hopefully later this month.”
Some 200km south, near the village of Dubicze Cerkiewne, a spokeswoman for Poland’s border service said officials had observed Belarusian soldiers encouraging “very aggressive” migrants to throw rocks and fireworks at Polish guards.
Of some 34,000 attempted border crossings recorded this year in Poland, about 5,500 have taken place this month.
Mr Morawiecki told Germany’s Bild tabloid on Thursday that, by guarding its border, Poland was defending the whole of Europe.
"We Poles are determined to protect our border by all means, which is also the eastern border of Europe and Nato, " he said.
Mr Morawiecki repeated his claim that Minsk's strategy – of attracting migrants from the Middle East and corralling them at the Polish border – had been masterminded in Moscow, something both Belarus and Russia deny.
The Polish leader said the border crisis could yet require the activation of Nato’s article 4, requiring Poland’s alliance members to come to its aid.
"It is possible that the crisis on the border is aimed at distracting attention from the new military attacks that Putin is preparing against Ukraine, " he added, saying "nothing can be ruled out".
Germany's acting chancellor, Angela Merkel, had a second round of telephone talks with Belarusian president Alexander Lukashenko on Wednesday, restating the need to allow full humanitarian access and return options for migrants.
On a trip to Warsaw on Thursday, her acting interior minister, Horst Seehofer, promised Berlin's "solidarity" with its eastern neighbour but dismissed reports that Germany was prepared to accept migrants camped on Poland's border.
“What we will not do is accept refugees and give in to pressure,” he said.