A second wave of migrants is expected to be sent back from Greece to Turkey on Friday as part of an EU deal to reduce the numbers reaching Europe.
The first group arrived in Turkey on Monday, but since then the process has stalled, partly due to a surge in last-minute asylum applications in Greece.
It is thought another two boats will arrive on Friday carrying migrants expelled from Greece under the EU deal.
Around 200 people, mainly Pakistanis, were sent back to Turkey on Monday.
Under the EU deal with Turkey, migrants who have arrived illegally in Greece after 20 March are expected to be sent back to Turkey if they do not apply for asylum or if their claim is rejected.
And for each Syrian migrant returned to Turkey, the EU is due to take in another Syrian who has made a legitimate request.
Of those being returned to Turkey on Friday, the non-Syrians will be taken to deportation centres while any Syrians will be taken to refugee camps to take the place of Syrian refugees who will be directly resettled in the EU.
But Turkey’s President Recep Tayyip Erdogan said on Thursday that his country will only implement this deal if the EU sticks to its side of the bargain:
- A further 3bn euros ($3.4bn)
- Galvanising Turkey’s EU accession process
- Granting Turks visa-free travel to the EU’s Schengen area by the end of June
“There are precise conditions. If the European Union does not take the necessary steps, then Turkey will not implement the agreement,” Mr Erdogan said in a speech at his presidential palace in Ankara.
But German Chancellor Angela Merkel, whose country has taken in the bulk of the asylum seekers so far, was more upbeat.
“I am very happy today. However, I know that we have not yet completed all the tasks before us,” she told a news conference during a meeting with French counterpart Francois Hollande in eastern France.
- Migrants await fate on Lesbos
- The crisis in seven charts
- Short journey back – BBC correspondents in Turkey and Greece
The arrangement has alarmed rights groups, who say Turkey is not a safe country for migrants.
Amnesty International has accused Turkey of illegally returning Syrians to their homeland, something Turkey denies.
On Thursday Amnesty also raised the alarm on the lack of information and help available for migrants on the Greek islands.
People detained on Lesbos and Chios have virtually no access to legal aid, limited access to services and support, and hardly any information about their current status or possible fate,” said Amnesty Deputy Director for Europe Gauri van Gulik.
“The fear and desperation are palpable,” Reuters quoted her as saying.
“They show that in addition to Turkey not being safe for refugees at the moment, there are also serious flaws on the Greek side of the EU-Turkey deal.”
in 2016, up to 4 April
37% of 2016 arrivals are children
53% arrive on Lesbos
366 died on Turkey-Greece route
853,650 arrivals in 2015
Migrants on the Greek border with Macedonia clashed with police on Thursday, demanding that the border be reopened so they could continue with their journey that way.
One million migrants and refugees have entered the EU by boat from Turkey to Greece since last year.
Many are keen to travel to Germany and other northern EU countries, and experts have warned the deal could force them to take alternative, more dangerous routes.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.