European Council President Donald Tusk has warned illegal economic migrants against coming to Europe, during a new push to solve the EU migrant crisis.
He said illegal economic migrants were risking "lives and money" for nothing.
Tusk is visiting Greece and Turkey to try to secure agreement on reducing the flow of migrants travelling west.
More than 25,000 migrants have been left stranded in Greece by a tightening of border controls to the north, raising fears of a humanitarian crisis.
On Thursday, a group of migrants blocked a railway line on the Greek side of the border with Macedonia to protest at the restrictions.
The restrictions were imposed after Austria and several Balkan countries decided only to allow Syrian and Iraqi migrants across their frontiers.
The move effectively barred passage to thousands of people seeking to reach western Europe illegally, including Afghans as well as some more likely to be regarded as economic migrants.
Separately, thousands of migrants are living rough around the French port of Calais, hoping to be able to enter the UK.
UK Prime Minister David Cameron and French President Francois Hollande are expected to discuss the issue in talks today.
'Do not come to Europe'
After meeting Greek Prime Minister Alexis Tsipras in Athens on Thursday, Mr Tusk said he was appealing to "all potential illegal economic migrants", wherever they may be from.
"Do not come to Europe," he said. "Do not believe the smugglers. Do not risk your lives and your money. It is all for nothing."
He also said EU member states must avoid taking unilateral action to deal with the migrant crisis.