A senior European Union official carried a stark warning Thursday to the front lines of the migrant crisis, telling those seeking to flee poverty and unrest that Europe is no longer their answer.

“Do not come to Europe,” said Donald Tusk, president of the European Council, after meeting with the Greek prime minister in Athens. “Do not believe the smugglers. Do not risk your lives and your money. It is all for nothing.”

His comments reflect a wider retrenchment across Europe as nations step up border controls and other barriers to halt the unprecedented stream of asylum seekers, refugees and others from conflict zones such as Iraq and Syria.

In the past week, unrest has broken out among migrants stranded at the blockaded Macedonian border and at a makeshift camp being dismantled in France.

At the same time, NATO ships last month began patrols in the waters between Turkey and Greece — the main route for nearly 1 million migrants in the past year — in efforts to confront smuggling groups.

Tusk spoke in Athens after meeting with Greek Prime Minister Alexi Tsipras as up to 10,000 migrants piled up at Greece’s northern border with Macedonia, which has allowed only a trickle of people to cross.

On Wednesday, the European Union announced plans for an emergency 700 million euros ($763 million) in humanitarian aid, but officials across the continent are struggling to manage the largest immigration crisis on European soil since World War II.

In France, meanwhile, British Prime Minister David Cameron and French President François Hollande held talks seeking to contain the migrant crisis in northern France, where thousands of refugees are camped in squalid conditions just over two hours from both London and Paris.

The meeting took on added dimensions after France’s economy minister was quoted as saying that border controls could be lifted if Britain leaves the European Union, opening up a potential path for migrants seeking to cross the English Channel.

The talks in Amiens come days after French authorities began demolishing sections of the infamous “Jungle” encampment in Calais, home to an estimated 4,000 migrants from North Africa and the Middle East who are mostly seeking to reach Britain.

Riots have broken out between local police and migrants, some of whom sewed their mouths shut to dramatically protest mass evictions and the destruction of their makeshift dwellings.

Paris has requested additional aid from London in managing the crisis. In advance of the summit, Harlem Désir, France’s secretary of state for European Affairs, announced Thursday on RFI radio that the figure will now include an additional 20 million euros ($21.8 million) — on top of the current 60 million euros ($65.4 million).

The extra funds, he said, will help with “securing the access area to the tunnel and Calais port area” as well as “the fight against smuggling networks.”

Emmanuel Macron, the French economy minister, told the Financial Times that if Britain votes to leave the European Union, the French could end a deal that allows border controls to be carried out in France. Britain is expected to hold a referendum in June on whether to leave the 28-nation bloc.

“The day this relationship unravels, migrants will no longer be in Calais,” he said.

In a statement, Cameron echoed these concerns, calling the meeting “an opportunity to discuss how we can work even more closely together to keep our people safe.”

“I am convinced that the U.K.’s membership of the E.U. gives us greater security and greater capacity to project power globally,” he said.

Campaigners for a British exit, dubbed “Brexit,” dismissed the warnings as “propaganda.”

Bernard Jenkin, a Conservative lawmaker who is campaigning for Brexit, told BBC Radio 4’s “Today” program: “What we are having now is propaganda being produced by other European governments at the request of the prime minister to try to scare people.”

(c) 2016, The Washington Post · James McAuley