European Union leaders were gathering Thursday for a potentially divisive debate amid a migration crisis on whether to create a new border guard with powers to unilaterally deploy to countries unable to manage their borders.

Hundreds of thousands of migrants have entered Greece this year, hopping into Europe through the Greek islands from Turkey, but barely a quarter of them have been registered or fingerprinted.

With pressure building on nations further north — preferred migrant destinations like Germany or Sweden — the EU's executive arm has floated the border and coast guard plane to force Greece to act.

But the plan touches at the very heart of national identity — a country's right to decide what kinds of forces can be allowed on its territory — and Greece, Italy, Croatia and Hungary are likely to oppose it.

Before chairing the summit in Brussels, European Council President Donald Tusk acknowledged that the plan is a "most controversial idea" but said that the passport-free Schengen area must be bolstered.

"Europe cannot remain vulnerable when Schengen states are not able to effectively protect their borders," he told reporters.

But he warned that any failure of this new border and coast guard agency project might only lead to "another, but I am afraid, equally painful solution."

The new agency would have a standing reserve force of at least 1,500 border guards that could be sent to EU border crossings within three days, rather than relying on time-consuming calls to nations for volunteers during emergencies.

The proposal is just the start of a potentially long and divisive legislative process. Still, the EU has planned a 2016 budget for the agency of 238 million euros ($260 million).

According to the International Organization for Migration, more than 920,000 people have entered the EU so far this year. The influx has overwhelmed national border guards and reception capacities, notably in Greece, where some 770,000 migrants have arrived, most of them from Turkey.