Migrants are being sent to Europe as a campaign of "hybrid warfare" to force concessions to its neighbours, the EU president has claimed.

An influx of hundreds of thousands of people is a "weapon" and a "political bargaining chip" used by the EU's neighbours who want to harm the continent, Donald Tusk said.

He made the comments as the European Union announced it would give an extra €1bn in aid and dangled the offer of visa-free travel in an effort to force Turkey to close its borders.

Sources said his remarks were not aimed specifically at Turkey, but he was thinking of leaders across the region, including the late Libyan dictator Muammar Gaddafi, who threatened to turn Europe "black" unless it handed over billions in aid.

Mr Tusk also appeared to ridicule German Chancellor Angela Merkel's decision to grant asylum to hundreds of thousands of Syrians without conditions.

It was a "beautiful moral gesture", but she now faces an "exam in responsibility for the protection of the European political community and its external borders", he said.

There is mounting frustration in Brussels at President Recep Tayyip Erdogan's refusal to seal Turkey's coasts and border with Greece.

Police have stopped just one in seven of the 350,000 people who have crossed since January on a major transit route for those fleeing Syria for the EU.

Mr Tusk, the president of the European Council, told MEPs that "many of our neighbours look with satisfaction at our troubles", and were prepared to extract favours to halt the flow.

"For us, refugees are specific people, individuals, who expect our help. There are forces around us, however, for whom the wave of refugees is just dirty business or a political bargaining chip.

"We are slowly becoming witnesses to the birth of a new form of political pressure, and some even call it a kind of a new hybrid war, in which migratory waves have become a tool, a weapon against neighbours.

"This requires sensitivity and responsibility on our side."

Under a deal struck in Brussels, the EU said it would "step up" the resettlement of refugees from Turkey and help to reinforce the Turkish coast guard to stop the flow of boats to Greece.

It has been suggested that as many as half a million people could be moved from Turkey, but the document does not specify numbers.

Turkey says it has spent more than €6.5bn on providing support to Syrian refugees and has demanded more help.

However, the deal also makes it clear that Turkey's long-term goal of visa-free travel for 75 million people to Europe depends on it fixing its border. It also hints that refusal would put at risk talks for the country to win full EU membership.

Telegraph.co.uk