• A Polish soldier builds a fence on the border between Poland and Belarus near the village of Nomiki, Poland

A Polish soldier builds a fence on the border between Poland and Belarus near the village of Nomiki, Poland (Photo : REUTERS/Kacper Pempel)

Polish soldiers were building a fence on the border with Belarus on Thursday, as the European Union's largest eastern member takes steps to curb illegal border crossings despite criticism that some migrants are being treated inhumanely.

Brussels has accused Belarusian President Alexander Lukashenko of using migrants as part of a "hybrid war" designed to put pressure on the bloc over sanctions it has imposed, and building the wall is part of Poland's efforts to beef up border security on the EU's eastern flank.

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"Almost 3 km of fencing has been erected since yesterday," Defence Minister Mariusz Blaszczak said on Twitter, adding that almost 1,800 soldiers were supporting the border guard.

Blaszczak said on Monday that a new 2.5 metre high solid fence would be built, modelled on the one built by Prime Minister Viktor Orban on Hungary's border with Serbia.

On Thursday Reuters saw soldiers next the frontier stringing wire through barbed wire to hook it to posts.

Poland has received sharp criticism over its treatment of a group of migrants who have been stuck on the Belarus border for over two weeks, living in the open air with little food and water and no access to sanitary facilities.

On Wednesday refugee charity the Ocalenie Foundation said 12 out of 32 migrants stuck on the border were seriously ill and one was close to death.

"No fence or wire anywhere in the world has stopped any people fleeing war and persecution," said Marianna Wartecka from the foundation who was at the border on Thursday.

Poland says responsibility for the migrants lies with Belarus. The prime minister said this week that a convoy of humanitarian offered by Poland had been refused by Minsk.

Surveys show that most Poles are against accepting migrants, and Poland's ruling nationalists Law and Justice (PiS) made a refusal to accept refugee quotas a key plank of its election campaign when it swept to power in 2015.

An IBRiS poll for private broadcaster Polsat on Wednesday showed that almost 55% of respondents were against accepting migrants and refugees, while over 47% were in favour of a border wall.

"Our country cannot allow such a large group of people to break our laws," said Emilia Krystopowicz, a 19-year-old physiotherapy student, in Krynki, a village next the border.

Belarusian President Alexander Lukashenko has accused Poland and Lithuania of fuelling the migrant issue on the borders.