The row broke out as EU leaders prepared to meet in Brussels on Thursday for a summit on the migration challenges facing the bloc.  

At the heart of the dispute — and topping the meeting's agenda — is an EU quota system to distribute tens of thousands of refugees across EU member nations.

In 2015, the EU introduced a scheme to relocate 160,000 refugees from overburdened frontline states Greece and Italy. But so far only 32,000 people have been moved to new homes.

Hungary, Poland, Slovakia and the Czech Republic fiercely opposed the plan from the start and have repeatedly refused to accept asylum seekers. That's prompted the European Commission, as well as wealthier states like Germany and Sweden, to call for a permanent mechanism for refugee-sharing.

But in a pre-summit letter to leaders, European Council President Donald Tusk criticized mandatory relocation as "ineffective" and "highly divisive."

He added that the EU should instead focus its efforts on securing borders, and "look at what has — and what has not — worked over the past two years, and draw the necessary lessons."

Tusk's letter provoked an angry response from the EU's executive arm, one of the scheme's main backers.

Migration Commissioner Dimitris Avramopoulos hit back at Tusk, saying that his intervention was "unacceptable" and "anti-European."

"It ignores all the work we have done during the past years…Europe without solidarity cannot exist," Avramopoulos said. "It is a duty — moral and legal — to protect refugees."

Following the exchange, one EU diplomat said the upcoming summit would likely yield "a very lively and maybe controversial debate."

Another diplomat told Reuters news agency that discussions on the issue were like "fighting trench warfare."

No concrete decisions will come out of the meeting, but EU leaders are expected to arrive at a position on mandatory quotas by June 2018.

nm/sms (AFP, Reuters, AP)