(CNN) As many as 14,000 migrants are stranded in dire conditions in a muddy transit camp on the Greek border after Macedonia finally closed its doors to refugees, sealing shut the main migration route to Western Europe.

Humanitarian workers described desperate scenes unfolding Thursday at the overwhelmed camp at Idomeni, Greece, where a severe lack of shelter was made worse by a month's worth of rain falling the previous day, according to the U.N. refugee agency.

Many at the camp, about 40% of whom are children, are suffering respiratory problems, the agency added.

Greek officials, who want to clear the camp due to the unsafe conditions, now face the challenge of trying to move thousands of desperate migrants.

"They are living in mud," said George Kiritsis, spokesman for the Greek government's refugee crisis response team.

"It's hard to feed and hard to support people medically."

The crowds were left stranded by Macedonia's decision to close its border to migrants Wednesday.

The closure followed similar moves by other countries along the so-called western Balkan route, the overland path taken by hundreds of thousands of migrants who have entered Europe from Turkey through Greece, on their trek to desirable northern European "destination countries" such as Germany and Sweden.

German Chancellor Angela Merkel criticized the move in an interview with a German public radio station Thursday, describing it as a unilateral action that put undue pressure on Greece, where as many as 42,000 migrants are now stuck without an onward route through Europe.

In what officials said was a collective decision, Slovenia, Croatia and Serbia all closed their borders from midnight Tuesday to migrants without visas or proper authorization to continue along the route.

"Serbia cannot afford to become a collection center for refugees," Serbia's Ministry of Internal Affairs told CNN in an email.

The move brings about an end to the "wave-through" approach which had arisen since the start of Europe's migrant crisis, and a return to regular border controls, officials said.

Merkel told German radio station MDR that closing the route did not solve the problem, and would only put Athens "in a very difficult situation."

She reiterated her calls for a unified European response to the crisis.

Increasing numbers stranded in Greece

The closures had been signaled at the end of an emergency summit between EU heads of government and Turkey on Monday, when leaders declared that the western Balkans route would be sealed to migrants, bringing an end to irregular migration into Europe.

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Governments had been tightening border restrictions along the route for weeks prior to the move, with Macedonia refusing entry to all but Syrians and Iraqis, and allowing only a few hundred or so through a day.

But on the other side of the Greek-Macedonia border, migrants have continued to flood in from Turkey, at an average rate of 1,800 people each day last month.

More than 3,300 arrived in Greece by sea on Wednesday alone, the U.N. refugee agency said.

With migrants refused onward travel through Europe, observers have warned that Greece risks becoming a huge refugee camp

'Soft approach' to clearing overwhelmed camp

They fear scenes such as the ones unfolding at Idomeni, where as many as 14,000 migrants are stranded at a camp only equipped to house 2,400, the U.N. refugee agency said.

People line up for food near the Greek village of Idomeni the day after Macedonia sealed the border to migrants.

The camp has insufficient accommodation, hygiene and lighting, and women and girls there are particularly vulnerable to abuse when the sun sets, said the International Rescue Committee.

Kiritsis, the spokesman for the Greek government's Coordinating Body for Refugee Crisis Management, said officials were taking a "soft approach" in encouraging migrants to voluntarily shift to the south of the country where there were better facilities.

That tactic was better than attempting to forcibly remove them, he said.

"Half are women and children. We won't do that," he said. "We try with flyers and translators. We understand that it will take some time to sink in that they are not moving."

Several buses left Idomeni transporting migrants back to Athens Wednesday, workers at the camp said, but most remained, uncertain of what their options were now.

'Bold' EU-Turkey plan to tackle crisis

At the conclusion of Monday's emergency EU-Turkey summit, leaders announced a bold new proposal to tackle the crisis

Under the suggested "one-for-one" deal, Ankara would take back all migrants who leave Turkey's shores for Europe in the future, on the condition that one Syrian refugee is resettled in Europe for every Syrian returned to Turkey.

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The plan would also see the EU provide Turkey with billions in additional funding for refugees, speed up talks on Turkey joining the EU and accelerate the lifting of visa requirements for Turkish citizens in Europe.

Officials still need to hammer out the details of the proposal, elements of which were harshly criticized by international humanitarian groups, before it is sent for approval by EU leaders next week.

European leaders also pledged to provide "massive humanitarian assistance" to Greece -- which is already struggling with a debt crisis -- to help it respond to the unfolding humanitarian crisis within its borders. NATO ships have also been deployed in an effort to smash smuggling networks operating in the Aegean Sea.

Photos: Europe's migration crisis in 25 photos A woman cries after being rescued in the Mediterranean Sea about 15 miles north of Sabratha, Libya, on July 25, 2017. More than 6,600 migrants and refugees entered Europe by sea in January 2018, according to the UN migration agency , and more than 240 people died on the Mediterranean Sea during that month. Hide Caption 1 of 26 Photos: Europe's migration crisis in 25 photos Refugees and migrants get off a fishing boat at the Greek island of Lesbos after crossing the Aegean Sea from Turkey in October 2015. Hide Caption 2 of 26 Photos: Europe's migration crisis in 25 photos Hide Caption 3 of 26 Photos: Europe's migration crisis in 25 photos Migrants step over dead bodies while being rescued in the Mediterranean Sea, off the coast of Libya in October 2016. Agence France-Presse photographer Aris Messinis was on a Spanish rescue boat that encountered several crowded migrant boats. Messinis said the rescuers counted 29 dead bodies -- 10 men and 19 women, all between 20 and 30 years old. "I've (seen) in my career a lot of death," he said. "I cover war zones, conflict and everything. I see a lot of death and suffering, but this is something different. Completely different." Hide Caption 4 of 26 Photos: Europe's migration crisis in 25 photos Authorities stand near the body of 2-year-old Alan Kurdi on the shore of Bodrum, Turkey, in September 2015. Alan, his brother and their mother drowned while fleeing Syria. This photo was shared around the world, often with a Turkish hashtag that means "Flotsam of Humanity." Hide Caption 5 of 26 Photos: Europe's migration crisis in 25 photos Migrants board a train at Keleti station in Budapest, Hungary, after the station was reopened in September 2015. Hide Caption 6 of 26 Photos: Europe's migration crisis in 25 photos Children cry as migrants in Greece try to break through a police cordon to cross into Macedonia in August 2015. Thousands of migrants -- most of them fleeing Syria's bitter conflict -- were stranded in a no-man's land on the border. Hide Caption 7 of 26 Photos: Europe's migration crisis in 25 photos The Kusadasi Ilgun, a sunken 20-foot boat, lies in waters off the Greek island of Samos in November 2016. Hide Caption 8 of 26 Photos: Europe's migration crisis in 25 photos Migrants bathe outside near a makeshift shelter in an abandoned warehouse in Subotica, Serbia, in January 2017. Hide Caption 9 of 26 Photos: Europe's migration crisis in 25 photos A police officer in Calais, France, tries to prevent migrants from heading for the Channel Tunnel to England in June 2015. Hide Caption 10 of 26 Photos: Europe's migration crisis in 25 photos A migrant walks past a burning shack in the southern part of the "Jungle" migrant camp in Calais, France, in March 2016. Part of the camp was being demolished -- and the inhabitants relocated -- in response to unsanitary conditions at the site. Hide Caption 11 of 26 Photos: Europe's migration crisis in 25 photos Migrants stumble as they cross a river north of Idomeni, Greece, attempting to reach Macedonia on a route that would bypass the border-control fence in March 2016. Hide Caption 12 of 26 Photos: Europe's migration crisis in 25 photos In September 2015, an excavator dumps life vests that were previously used by migrants on the Greek island of Lesbos. Hide Caption 13 of 26 Photos: Europe's migration crisis in 25 photos The Turkish coast guard helps refugees near Aydin, Turkey, after their boat toppled en route to Greece in January 2016. Hide Caption 14 of 26 Photos: Europe's migration crisis in 25 photos A woman sits with children around a fire at the northern Greek border point of Idomeni in March 2016. Hide Caption 15 of 26 Photos: Europe's migration crisis in 25 photos A column of migrants moves along a path between farm fields in Rigonce, Slovenia, in October 2015. Hide Caption 16 of 26 Photos: Europe's migration crisis in 25 photos A ship crowded with migrants flips onto its side in May 2016 as an Italian navy ship approaches off the coach of Libya. Passengers had rushed to the port side, a shift in weight that proved too much. Five people died and more than 500 were rescued. Hide Caption 17 of 26 Photos: Europe's migration crisis in 25 photos Refugees break through a barbed-wire fence on the Greece-Macedonia border in February 2016, as tensions boiled over regarding new travel restrictions into Europe. Hide Caption 18 of 26 Photos: Europe's migration crisis in 25 photos Policemen try to disperse hundreds of migrants by spraying them with fire extinguishers during a registration procedure in Kos, Greece, in August 2015. Hide Caption 19 of 26 Photos: Europe's migration crisis in 25 photos A member of the humanitarian organization Sea-Watch holds a migrant baby who drowned following the capsizing of a boat off Libya in May 2016. Hide Caption 20 of 26 Photos: Europe's migration crisis in 25 photos A migrant in Gevgelija, Macedonia, tries to sneak onto a train bound for Serbia in August 2015. Hide Caption 21 of 26 Photos: Europe's migration crisis in 25 photos Migrants, most of them from Eritrea, jump into the Mediterranean from a crowded wooden boat during a rescue operation about 13 miles north of Sabratha, Libya, in August 2016. Hide Caption 22 of 26 Photos: Europe's migration crisis in 25 photos Refugees rescued off the Libyan coast get their first sight of Sardinia as they sail in the Mediterranean Sea toward Cagliari, Italy, in September 2015. Hide Caption 23 of 26 Photos: Europe's migration crisis in 25 photos Local residents and rescue workers help migrants from the sea after a boat carrying them sank off the island of Rhodes, Greece, in April 2015. Hide Caption 24 of 26 Photos: Europe's migration crisis in 25 photos Investigators in Burgenland, Austria, inspect an abandoned truck that contained the bodies of refugees who died of suffocation in August 2015. The 71 victims -- most likely fleeing war-ravaged Syria -- were 60 men, eight women and three children. Hide Caption 25 of 26 Photos: Europe's migration crisis in 25 photos Syrian refugees sleep on the floor of a train car taking them from Macedonia to the Serbian border in August 2015. How to help the ongoing migrant crisis Hide Caption 26 of 26

Kiritsis said Athens was holding off finalizing its strategy to respond to the crisis until the EU-Turkey plan was confirmed.

Greek officials feared that human traffickers could step up their activities before the EU-Turkey plan was implemented, he said.

More than 1 million arrivals

European leaders are grappling with the biggest refugee crisis since World War II, with more than 1 million people having entered EU territory since the start of 2015.

The majority have come by using trafficking networks to cross the Aegean Sea, which separates Turkey and Greece, with more than 400 migrants dying in that crossing so far this year.

Most of the migrants are from Syria, where the civil war has created more than 4 million refugees.