Three Afghan migrants drowned Monday while trying to cross a river on the Greek border with Macedonia, Macedonian state TV MRT reported, quoting police.
The two men and a woman were part of a 26-member group sneaking through the border, which Macedonia sealed to migrants last week, causing a massive backlog of people on the Greek side.
The drowning occurred in Suva Reka, near the village Moin, 4 kilometres west of the refugee camp at Idomeni. The small river had swelled after days of relentless rain in the region.
Thousands of migrants, mostly Syrian and Iraqi refugees, have been stranded at Idomeni after Balkan countries first restricted, then stopped the flow of migrants toward wealthy EU countries.
Aid organizations have voiced concern that stifling the regulated migration flow could lead to the smuggling of people and potentially dangerous attempts at sneaking across borders.
In the Aegean Sea, meanwhile, rescuers were searching for eight migrants missing after their boat overturned in the rough sea early Monday morning.
Greek coastguard spokesman Nikos Langadianos told national TV that five people were rescued near the island Kos as one vessel each from the Greek and Italian coastguard, along with a helicopter, searched the sea for survivors.
Greek Aegean islands last year became the main gateway for migrants, including Syrian and Iraqi refugees, trying to reach Europe from Turkey.
At least 455 have drowned in the Aegean Sea this year alone, according to data from the International Organization for Migrations.
More than 995,000 people have made the eastern Mediterranean crossing since 2015, 139,000 of them since the start of the year, the UN refugee agency (UNHCR) reports.
From Greek islands, the people are ferried to Athens, then had typically continued their trip north and west, through Macedonia, Serbia, Croatia, Slovenia and Austria.
Countries on the Balkan route have been gradually restricting the flow, first stopping all but Syrians and Iraqis and, eventually, starting March 9, barring entry to all.
The European Union is now negotiating a deal with Turkey to also stem the number of migrants and asylum seekers who attempt to cross the Aegean Sea.
The deal, which EU officials hope to finalize at a summit in Brussels this week, had foreseen Ankara taking back any new migrants arriving in Greece, while the EU would agree to directly resettling an equal share of Syrians living in Turkey.
The plan is meant to discourage migrants from putting their lives in the hands of smugglers, but it has faced a barrage of criticism from human rights advocates and high-ranking UN officials, who have called the approach inhumane and illegal.
On Monday, Spanish Foreign Minister Jose Manuel Garcia-Margallo joined the critics' ranks, saying that his country is seeking changes to the "unacceptable" agreement.
"Spain will only accept ... an agreement that is coherent, compatible and respectful of international law and extraordinarily respectful of the human rights of people who have to flee from their country," he said in Brussels on the margins of an EU foreign ministers' meeting.
He noted that any "collective expulsions" by the EU of migrants would be particularly problematic.
Madrid has given a mandate to its EU ambassador to seek a correction of the legal texts, Garcia-Margallo said. He added that anyone who arrives on European soil should be allowed to file an asylum request that should be assessed individually, with a right to appeal.
The deal with Turkey is also expected to require legislative changes in both Turkey and Greece. Garcia-Margallo said Ankara has "given assurances" that it will adopt measures to ensure the "absolute protection of refugees."
Concerns about the deal have also been raised in other European capitals.
Bulgarian Prime Minister Boyko Borisov has demanded that his country be covered by any migration deal with Turkey, which it neighbours. French President Francois Hollande, meanwhile, has warned against the bloc rushing into concessions to strike a deal with Turkey.
"A few open questions still have to be settled in these days," German State Minister Michael Roth acknowledged during the foreign ministers' talks in Brussels. "But Turkey also needs the EU. It's not just the EU needing Turkey."