Thousands of migrants, mostly from the Middle East, have crossed or attempted to cross the EU border from Belarus in recent months.
A fifth migrant has died at the border between Poland and Belarus, border guard officials have confirmed.
Polish authorities said it happened as they detained a group of migrants from Iraq near the Belarusian border on Thursday night.
"One of the men, despite being resuscitated by the patrol and the ambulance team, died (probably of a heart attack)," the Polish Border Guard said in a statement.
It brings the death toll to five people since the start of the migration crisis in the eastern European Union this summer.
On Sunday, four migrants were found dead at the border, according to both Belarusian and Polish officials.
Thousands of migrants, mostly from the Middle East, have crossed or attempted to cross the EU border from Belarus into Latvia, Lithuania, and Poland in recent months.
The EU has accused Belarusian President Alexander Lukashenko of encouraging migrants to illegally cross the borders in retaliation against Brussels sanctions on Minsk.
Poland has sent thousands of soldiers to the 400-kilometre-long border and built a long section of barbed wire fence, while also introducing a local state of emergency.
On Monday, border guards said they had recorded 3,500 successful and unsuccessful border crossings in August, with around 5,000 more attempts in September alone.
"We are trying to help and save the lives and health of all illegal migrants who have crossed the border into Poland and have been found in time," Polish Prime Minister Mateusz Morawiecki wrote in a Facebook post on Friday.
But critics of the government have accused Warsaw of using heavy-handed methods against asylum seekers.
NGOs have also claimed that Poland is pushing people back from its border -- something that contravenes EU rules.
The EU's Commissioner for Home Affairs has called for an investigation into the deaths of the migrants and has urged Poland to allow the EU border agency Frontex, to help stabilise the area.
"We recognise that the Polish authorities are dealing with a difficult situation," a European Commission spokesman told a news conference on Friday.
"It is essential for Poland to carry out border management duties effectively. However, this should not come at the expense of human life."
"We urge member state authorities to ensure that people at the border are given the necessary care and assistance."
Meanwhile, the United Nations Human Rights Commissioner Michelle Bachelet has also urged Belarus's neighbours to protect people seeking asylum in Europe.
"I take this opportunity to remind all governments that under international law, no one should ever be prevented from seeking asylum or other forms of international protection," Bachelet told the UN Human Rights Council.
Michelle Bachelet also told the Geneva forum that Belarusian authorities had clamped down on fundamental freedoms, with arrests of activists and journalists "on what routinely appear to be politically-motivated charges".