Under the proposed deal, Ankara would agree to take back all migrants who leave Turkey's shores for Europe in the future, including those intercepted in its territorial waters, on the condition that one legitimate Syrian refugee is resettled in Europe for every Syrian returned to Turkey.
But international humanitarian groups have harshly criticized parts of the agreement, with a senior official from the U.N. refugee agency saying Tuesday that sending back refugees en masse would not be "consistent with European law."
"An agreement that would be tantamount to a blanket return to a third country is not consistent with European law, not consistent with international law," Vincent Cochetel, Europe regional director of the Office of the U.N. High Commissioner for Refugees, told reporters in Geneva, Switzerland.
EU chief: Irregular migration into Europe is 'over'
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. The proposal still requires details to be hammered out before being sent for approval by EU leaders next week.
"The days of irregular migration to the European Union are over," said Donald Tusk, president of the European Council -- as the group of 28 EU leaders is known -- at the end of this week's emergency summit in Brussels, Belgium.
He said Turkish Prime Minister Ahmet Davutoglu had "confirmed Turkey's commitment to accept the rapid return of all migrants coming from Turkey to Greece that are not in need of international protection."
"The EU will support Greece in ensuring comprehensive, large-scale and fast-track returns to Turkey," Tusk said.
A statement from EU heads of government said they agreed that "bold moves were needed" to break the business model of smugglers, highlighting the importance of a NATO anti-trafficking mission in the Aegean Sea that just expanded into Greek and Turkish territorial waters.
"We need to break the link between getting in a boat and getting settlement in Europe," the statement said.
Davutoglu said that his country, which hosts more Syrian refugees than any other, was motivated to enter into the arrangement primarily out of humanitarian concern.
"We don't want to see women and children dying in the Aegean Sea," he told reporters, according to Turkey's semiofficial Anadolu news agency.
Balkans migration route effectively closed
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. Most of the migrants are from Syria, where the civil war
has created more than 4 million refugees and displaced a further 6 million within the country.
The majority have come by using trafficking networks to cross the Aegean, which separates Turkey and Greece, before heading overland through the Balkans to Germany and other northern European countries.
The crossing is dangerous, with more than 400 migrants having died so far this year, according to the International Organization for Migration.
Recently, a number of countries along the Balkan migration route agreed to all but close their borders, leaving a bottleneck of desperate migrants stranded in Greece, already struggling with a debt crisis.
Tusk confirmed at the summit's end that EU leaders had decided to "end the 'wave-through approach'" through countries along the overland route to Western Europe.
"Irregular flows of migrants along the Western Balkans route have now come to an end," the EU heads of government said in a joint statement.
Making good on that vow, Serbia closed its southern borders, with Macedonia and Bulgaria, at midnight Tuesday, before Macedonia followed suit, sealing its border with Greece.
"We stopped accepting migrants at the border with Greece March 8 due to the fact that the Serbian government stopped accepting migrants on March 6 and the migrants couldn't reach their destination," a spokeswoman for the Macedonian Interior Ministry told CNN.
More than 11,000 people have been stuck on the Greece-Macedonia border in a transit camp at Idomeni designed for 1,500, according to Doctors Without Borders.
Serbia said it learned from Croatia that Slovenia, another EU member, would not receive migrants without valid visas and passports, effectively closing the Balkan route.
"Serbia cannot afford to become a collection center for refugees, so it will consolidate all measures with the European Union, and reciprocally apply them in its southern and eastern borders with Macedonia and Bulgaria," Serbia's Ministry of Internal Affairs told CNN in an email.
Those with no visas 'cannot enter the Schengen area'
In Slovenia, the Government Communications Office director confirmed that the country was closing its borders at midnight Tuesday.
"People who don't have proper documents -- i.e. people who don't have papers for Schengen -- cannot enter the Schengen area," Kristina Krajnc Plavsak told CNN. "We are strictly implementing Schengen rules."
She said the closure comes in coordination with Slovenia's neighbors and with other countries on the Balkan route, and it was not a unilateral decision.
Tusk said the EU would deploy "massive humanitarian assistance" to Greece to help it respond to the effects of the route's closure and would offer aid for the country to manage its external border.
Migrants were sent back from Greece to Turkey last week, Tusk said, in what he described as the "first visible step" of the Greek-Turkish bilateral agreement.
Concerns about the proposal
Other humanitarian organizations were quick to join the U.N. refugee agency in criticizing the proposal -- in particular, the mass return of refugees to Turkey.
Amnesty International said the plan showed an "alarmingly short-sighted and inhumane attitude" to the migrant crisis and would deal a "death blow to the right to seek asylum."
The statement said Amnesty opposed "the concept of a 'safe third country' in general, as this undermines the individual right to have asylum claims fully and fairly processed," and that there was "huge cause for concern" about sending migrants to Turkey, "given the current situation and treatment of migrants and refugees."
The statement attacked the "horse trading" concept of resettling a Syrian refugee in Europe for every compatriot sent back to Turkey, saying the proposal would make "every resettlement place offered to a Syrian in the EU contingent upon another Syrian risking their life by embarking on the deadly sea route to Greece."
"The idea of bartering refugees for refugees is not only dangerously dehumanizing, but also offers no sustainable long-term solution to the ongoing humanitarian crisis," said Iverna McGowan, head of Amnesty's European Institutions Office.
Amnesty also expressed concern about the closure of the Balkan migration route, which would "lead to thousands of vulnerable people being left in the cold with no clear plan on how their urgent humanitarian needs and rights to international protection would be dealt with."
The International Rescue Committee lauded the meeting in Brussels but warned that "closing all of Europe's borders without offering alternative routes to safety will not work."
"In fact," the organization said, "the only winners will be the smugglers, as people take more elaborate and more dangerous routes to safety."
U.N. refugee agency spokesman William Spindler said refugees should be returned to a third country only if certain safeguards were in place, such as a protections against "refoulement," a legal term used to describe returning asylum seekers somewhere they would be at risk.
Spindler called for the details of these safeguards to be clarified before the proposal was next put to EU leaders at a crisis meeting scheduled for March 17.
More funding to Turkey
Late last year, the European Union and Turkey agreed to a joint action plan in response to the migrant crisis. European leaders agreed to pay Turkey 3 billion euros ($3.3 billion) to support its refugee population and target people-smuggling networks -- a mission that has seen NATO warships deployed to the eastern Mediterranean
Tusk said that "despite good implementation" of that plan, it had failed to reduce the migrant flow sufficiently and that extra steps were necessary.
The new proposal would focus on speeding up the disbursement of the 3 billion euros already pledged to Turkey as well as providing new funding to alleviate the crisis. Turkey requested an extra 3 billion euros at this week's summit, according to European Parliament President Martin Schulz.
The EU would bear the cost of returning the migrants to Turkey under the proposal.
Syrian refugees settled in the EU under terms of the deal would be distributed among member states "within the framework of the existing commitments," a joint statement from the EU heads of government said.
Last year, the EU agreed to resettle 160,000 refugees, but less than 1,000 have been processed so far.