EU leaders trying to finalise a deal with Turkey on the migrant crisis have warned of the difficulties they face at the start of a summit in Brussels.
German Chancellor Angela Merkel said negotiations would be "complicated" but the direction was clear.
The proposed deal would see all migrants travelling to Greece from Turkey sent back.
In return the EU might offer Turkey incentives, including financial aid and visa-free access to Schengen countries.
The aim is to establish a joint position of all 28 EU member states before talks with the Turkish prime minister on Friday.
But Lithuanian President Dalia Grybauskaite warned that the plan to return people to Turkey was "on the edge of international law" and difficult to implement. And Belgian Prime Minister Charles Michel said he could not accept negotiations that looked like blackmail.
Since January 2015, a million migrants and refugees have entered the EU by boat from Turkey to Greece. More than 132,000 have arrived this year alone.
Tens of thousands are now stuck in Greece as their route north has been blocked.
"Germany will negotiate intensively because we have to improve the humanitarian situation in Greece," Mrs Merkel said.
Dutch Prime Minister Mark Rutte said if a deal could be reached the influx of migrants from Turkey to the Greek islands could be stopped "in three to four weeks".
Under initial proposals, for each Syrian migrant returned to Turkey, a different Syrian would be resettled in the EU directly from the country.
In return, the EU would double financial aid to Turkey promised last year, make a fresh push on talks over Turkey's eventual membership of the EU and offer visa-free travel to Europe's Schengen states.
However, those proposals have since been watered down, lowering expectation on greater financial help and talks on EU membership and linking visa-free travel to 72 conditions to which Turkey must agree.
EU member Cyprus has threatened to veto a deal. The Greek Cypriot government is not recognised by Turkey.
The Spanish foreign minister has said Spain will oppose the "blanket return" of refugees to Turkey.
Stressing what he called the UK's special status in the EU, Prime Minister David Cameron said he would not be offering visa-free access to Turkey. What mattered was "busting the business model of the people smugglers".
A number of EU countries have raised concerns about what is on offer to Turkey amid a clampdown by the Ankara government on academics and journalists.
German magazine Der Spiegel said on Thursday it had withdrawn its Istanbul correspondent, Hasnain Kazim, after the authorities refused to renew his press accreditation.
EU foreign policy chief Federica Mogherini said the talks with Turkey would not just focus on migration.
"We have the internal situation in Turkey we are working on, being it the human rights and the rule of law issues, being it the necessary process with the Kurds to reopen spaces for peace," she said.
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.