VALLETTA, Malta (AP) — Passport free travel in Europe is under threat as countries reintroduce border controls, toughen security and build fences in response to the biggest refugee emergency the continent has seen in decades.

The Schengen open-borders zone uniting 30 countries is a pinnacle of European achievement. It underpins the EU economy, allowing goods, services and people to cross frontiers without checks.

But Sweden says security at its borders cannot be assured and announced that checks were resuming on Thursday, while tiny Slovenia has begun erecting a fence to stem the flow of people from Croatia, the second nation, after Hungary, to resort to such a measure.

These uncoordinated and unilateral actions in response to unpredictable movements of thousands of people have raised fresh doubts about whether the passport free area can survive the migrant challenge.

“Saving Schengen is a race against time,” EU Council President Donald Tusk warned on Thursday after a migration summit with European and African leaders in Malta.

He cited individual moves by Germany, Sweden, Slovenia and others in response to what they see as threats to their border security from the tens of thousands of migrants streaming in from Africa, the Middle East and Asia.

“Without effective control of our external borders, Schengen will not survive,” Tusk said.

The two-day summit on the Mediterranean island was meant to focus on how to send back to Africa those who don’t qualify for asylum and discourage others from attempting the risky journey across the seas in search of a better life.

The leaders did sign up to an action plan of short and longer term measures to halt the flow of Africans coming to Europe and steps to send back those who don’t qualify for asylum.

They also signed on to an emergency package of migration aid worth 1.8 billion euros ($1.9 billion) that select African nations will be able to use.

Then the Europeans quickly huddled for informal talks about how to cope with their refugee emergency. The EU estimates that 3 million more people could arrive in Europe seeking sanctuary or jobs by 2017.

Many people are arriving in Europe via Turkey, and Tusk announced that the EU hopes to host a summit with Turkey in Brussels by the end of this month.

The EU is offering Turkey — which is hosting more than 2 million Syrian refugees — a package of sweeteners for it to crack down on border security, including billions of euros and speeding up its EU membership talks.

Turkey has asked to be present at high-level EU meetings, and the summit is being described by EU officials as an “easy deliverable” to get Ankara to boost border security.

The mass influx has overwhelmed European border authorities and countries simply to do not have the capacity to accommodate everyone.

The leader of Sweden — with the highest number of migrants per capita in Europe — defended his country’s move on border controls.

“When our authorities tell us we cannot guarantee the security and control of our borders, we need to listen,” Prime Minister Stefan Lofven said.

Lofven said his EU partners understand the decision, and he called for an overhaul of the rules governing Europe’s passport-free area.

“We need another system. That is obvious,” he said.

Tensions were also high Thursday in southeastern Europe, as Slovenia continued work on a razor-wire fence on its border with Croatia to hold back the migrant influx.

The two countries have a long-standing territorial dispute dating back to the wars in Yugoslavia in the 1990s, and Croatia believes the border fence is encroaching on its soil. Their leaders met in Malta to try to calm the row.

Slovenia says it is being overwhelmed by the arrival of more than 180,000 asylum-seekers moving toward Western Europe since mid-October.

Despite the influx and pledges of action, EU nations have been slow to move. A plan to share 160,000 refugees from Italy and Greece has barely got underway, with around 130 having left for other host nations.

“If we keep going at this rate we will have relocated 160,000 people in 2101,” European Commission President Jean-Claude Juncker said.