European Council President Donald Tusk is due to visit Greece and Turkey as he tries to find a common approach to Europe’s worsening migration crisis.
Mr Tusk has said reducing the number of migrants travelling from Turkey to the Greek islands is key to avoiding a humanitarian disaster.
More than 25,000 migrants are now stranded in Greece as border controls further north are tightened.
Migration is also due to be discussed at a UK-French summit on Thursday.
UK PM David Cameron and French President Francois Hollande will hold talks in the French city of Amiens. Counter-terrorism and conflicts in Libya and Syria are also expected to be on the agenda.
Hours before the meeting, French Finance Minister Emanualle Macron said his country could allow migrants to travel unchecked to the UK if British voters chose to leave the European Union in a June referendum.
Authorities in the northern French port of Calais have been clearing part of a sprawling camp known as the Jungle, from where many migrants are trying to enter the UK illegally.
But Mr Macron said an exit vote in the referendum could end a bilateral deal allowing the UK to vet new arrivals on French territory.
“The day this relationship unravels, migrants will no longer be in Calais,” Mr Macron told the Financial Times newspaper.
Mr Tusk has been touring capitals in central and south-eastern Europe this week, trying to ease tensions caused by the migration crisis.
His visits are seen as laying the groundwork for an EU-Turkey summit on the issue on 7 March.
“Not for a moment can we stop our efforts to improve the co-operation with our neighbours, starting with Turkey,” Mr Tusk said in Vienna on Tuesday.
He said Europe was ready to grant “substantial financial support” to countries neighbouring war-torn regions such as Syria and Iraq.
“But at the same time we expect a more intensive engagement from our partners as an absolute precondition to avoid a humanitarian disaster. This will be the subject matter of my talks in Turkey this Thursday and Friday,” he said.
Turkey has expressed frustration at the lack of a common position from the EU on the crisis
BBC Europe Correspondent Chris Morris, who is on the Greek island of Lesbos where 75,000 migrants have already arrived so far this year, says the EU wants to see arrivals in Greece drop below 1,000 a day.
New figures suggest last year’s total of one million seaborne migrants arriving in Europe could be matched well before the end of the year.
The number of migrants stuck in Greece has soared, after Macedonia began restricting passage to all but a small number.
On Wednesday only about 200 Syrian and Iraqi refugees were allowed to cross, with thousands still stuck on the Greek side of the border. More are en route from Athens.
Earlier this week, the European Commission adopted plans to distribute €700m (£543m; $760m) of emergency humanitarian funding between 2016-18 to help tackle the crisis.
Under the plan, which still needs approval by the European Council and Parliament, EU aid agencies would for the first time work directly with the UN and other groups inside Europe, using monies usually allocated to emergencies outside the bloc.
Greece has asked the European Commission for nearly €500m in assistance to help care for 100,000 asylum seekers.
The International Organization for Migration (IOM) said nearly 129,500 migrants had arrived in Europe by sea so far in 2016, plus another 1,545 by land. It said 418 had drowned or were missing.
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.