BUCHAREST, Romania (AP) — European Council President Donald Tusk has warned just days before an EU summit that the 28-nation bloc could break up under strain from mass migration and a deal to keep Britain in the union.

Speaking in Bucharest, Tusk warned Monday: "This is a critical moment. It is high time we started listening to each other's arguments more than to our own."

He said it was "natural that positions harden closer to crunch time," three days before the EU summit, which will have "the two biggest challenges to the future of the European Union on the agenda, Britain's membership of the EU and migration crisis."

"The risk of break up is real," Tusk added. "This process is indeed very fragile. Handle with care! What is broken cannot be mended."

Romania joined the EU in 2007 and President Klaus Iohannis called for Britain to remain in the bloc. "The EU needs Britain and Britain needs the EU.... I expressed this on all occasions"

Iohannis said Romania would be flexible and was prepared to help find solutions for Britain to remain, but insisted that measures applied to Romanian citizens should be non-discriminatory. There are an estimated 175,000 Romanians working in Britain.

Tusk also said that the wave of migrants which have arrived in the past year "is stretching our union to its limits."

"It is most important is to stem the flow of migrants... we must increase humanitarian aid to migrants" in countries neighboring Syria.