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.