European leaders hailed a potential "breakthrough" in the migrant crisis after Turkey agreed to take back anyone who'd left its shores — a proposal harshly criticized as unrealistic or worse by rights groups.
The move came as thousands of refugees and migrants camped overnight in freezing conditions along Greece's border with Macedonia, hoping to cross but awaiting news.
Leaders met late into the night on Monday to address the crisis, which has seen nearly 142,000 refugees and migrants arrive in Europe so far this year.
Turkey already had said it would accept the return of migrants who'd made it to Greece.
The country went a step further by offering late Monday to take back anyone — migrant or refugee, Syrian or Algerian — who'd made it to Greece from its shores, according to statements from EU officials following the summit.
Turkey's proposal, though, came with conditions: That the EU would cover the cost of sending those people back and also take in an equivalent number of Syrians — a proposal dubbed "one-for-one" resettlement.
Turkey also requested quicker disbursement of 3 billion euros pledged to help Syrian refugees — and an additional 3 billion euros on top of that until 2018, according to an EU source.
The "one-for-one" resettlement — where EU member states would agree to take in a Syrian from Turkey to offset each Syrian sent back to Turkey under the proposal — is aimed at destroying smuggling networks, according to the source. It would show that paying smugglers for boat rides just results in being returned to Turkey, the source explained.
Prime Minister Ahmet Davutoglu said Turkey's "game-changing position" was "to discourage illegal migration" and "to help people who want to come to Europe through encouraging legal migration in a disciplined and regular manner."
It was not immediately clear how Turkey's plan would be put into practice — European Council President Donald Tusk said he would "work out the details" of the "bold" proposals in the coming weeks.
German Chancellor Angela Merkel called Turkey's offer a potential "breakthrough" that could break a "vicious cycle" of illegal migration — if the plan is realized.
The EU source acknowledged that logistics and details still need to be worked out — but said European leaders were keen to get the crisis under control and Turkey is seen as the key to that.
However, the proposal raised immediate questions over its viability and legality.
United Nations High Commission for Refugees Filippo Grandi said he was "deeply concerned" about any arrangement that would involve the "blanket return" of refugees.
"Turkey is already hosting nearly 3 million refugees and it is important for the international community to share this responsibility more widely," Grandi told European Parliament on Tuesday.
Blanket returns would be "plainly incompatible" with EU obligations, according to Human Rights Watch's Judith Sunderland, who added that she "cannot fathom" how a one-for-one swap would be put into practice.
"I think we're about as close to rock bottom as we've ever been in what has been a year and a half of scrambling by EU leaders to face up to an influx of asylum seekers and migrants," she told NBC News. "What we're seeing really is just the naked desperation by EU leaders to stem the flow and they're willing to throw rights completely overboard in the process."
Doctors Without Borders said the plan to send migrants back to Turkey was "no solution," while Amnesty International called it an "alarmingly short-sighted and inhumane" approach.
"EU and Turkish leaders have today sunk to a new low, effectively horse trading away the rights and dignity of some of the world's most vulnerable people," Amnesty said in a statement that also condemned the one-for-one proposal.
"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," the statement added.
In addition to hearing Turkey's ramped-up proposals, EU leaders also agreed Monday night to end a "wave-through" border policy in order to close the Western Balkans migration route.
Asylum claims are supposed to be registered and processed in the first EU nation an asylum-seeker arrives in — which in this crisis has overwhelmingly meant Greece. However, the influx of migrants and refugees has seen tens of thousands press onward in hopes of reaching richer EU nations such as Germany or Sweden.
Aid organizations warned that closing the Balkans route would strand thousands in Greece and place undue pressure on the already overburdened nation.
More than 12,000 refugees and migrants spent the night at the Greek border with Macedonia under driving rain, waiting to see if they would be allowed to cross.
Pregnant women and children slept on the ground in the cold, according to Amnesty International's Fotis Filippou.
"People slept in freezing mud, they have water in their blankets. Children are getting sick," he told NBC News from the Greek border town of Idomeni. "It's just horrible."
There were an estimated 15,000 people in Idomeni as of Tuesday afternoon, according to Doctors Without Borders — and the people are "just stuck here," Filippou added.
EU leaders pledged in their statement to stand by Greece "in this difficult moment" and to the "utmost to help manage" the situation, such as by providing further assistance in managing the borders and helping ensure "large scale and fast-track returns" to Turkey.
That did little to allay the concerns voiced by organizations such as Human Rights Watch, which said refugees "should not be used as bargaining chips" by Turkey or the EU.
Human Rights Watch said Turkey cannot be considered a "safe country" of asylum for refugees from places such as Syria, Iraq or Afghanistan — and questioned how rights could be protected during "mass expulsions" from Europe.
"Turkey has ratified the 1951 Refugee Convention, but is the only country in the world to apply a geographical limitation so that only Europeans can get refugee status there," the organization said in a statement. "It does not provide effective protection for refugees and has repeatedly pushed asylum seekers back to Syria."
Questions about the proposals also are being asked among the thousands of refugees and migrants stuck along the Greek border, according to Amnesty's Filippou.
"Everybody is asking about it.... They hear the rumors they hear that people might be sent back to Turkey," he said. "People are definitely worried. They made it all the way here in search of safety and protection. They know they can't get it here but they also don't see any way out."