BRUSSELS (AP) — European Union chief Donald Tusk is making clear that the bloc's 28 leaders are heading into a summit this week that could fundamentally alter the political course of the continent.

Negotiations with Britain are still finely balanced over whether the EU can make the necessary reforms for Prime Minister David Cameron to support remaining an EU member in a national referendum on the issue.

At the same time, the migrant crisis is quickly turning into a seminal event, pitting east against west and many against Greece.

"The migratory crisis we are currently witnessing is testing our union to its limits," Tusk said as he sought compromise on both issues ahead of Thursday's summit.