Hundreds of migrants streamed out of a camp near the northern Greek border yesterday to try to find a way around a border fence blocking their way into Macedonia, a Reuters witness said.

A large group of migrants marched away from the sprawling tent city near Idomeni, where at least 12,000 people have been stranded in filthy conditions since Macedonia and other nations blocked their route north along the so-called Western Balkan route, according to a Reuters TV crew.

Meanwhile, 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 kilometers 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.

More than 995,000 people have made the eastern Mediterranean crossing since 2015, 139,000 of them since the start of the year, the U.N. refugee agency (UNHCR) reports. 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.