Fresh infighting has broken out among top EU officials and member states over the resettlement of hundreds and thousands of newly-arrived migrants throughout the continent. For the first time a top EU official has spoken against Brussels’ proposed mandatory relocation plan.

European Council President and former Prime Minister of Poland, Donald Tusk, slammed the migrant resettlement plan – a project backed by German Chancellor Angela Merkel and French President Emmanuel Macron – as “ineffective” and “highly divisive.”

Tusk called for the urgent need to secure Europe’s outer borders to stem the unregulated influx of migrants.

Tusk’s remarks angered German Chancellor Merkel, the driving force behind the controversial relocation scheme. She reiterated her position calling upon eastern European countries to follow Germany’s example and accept more migrants. There can be no question of “selective solidarity” with the EU, Merkel said, pointing to Germany’s financial contribution to the economies of the eastern Europe.

EU migration czar Dimitris Avramopoulos attacked Tusk for daring to question Brussels policy towards migrants, labeling his latest criticism "anti-European."

The bitter row is intensifying just ahead of a major EU summit in Brussels scheduled for Thursday that seeks to resolve the contentious issue of redistributing migrants across the member states.

A senior official summed up the current mood in Brussels as “trench warfare,” German broadcaster Deutsche Welle reported. The broadcaster covered the latest phase of infighting over the EU’s controversial migrant relocation plan:

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."

Even as the leaders of the EU member states come together to deliberate on the issue, Brussels is suing Hungary, Poland, and the Czech Republic for refusing to open their borders to migrants.

These three countries form the biggest stumbling blocks in the path of EU’s open arms policy towards illegal immigrants. Eastern European politicians, like Hungarian Prime Minister Viktor Orbán, are at the forefront of resisting EU’s push for open borders for migrants, mostly fighting-age men from Arab and Muslim countries.

In October, Prime Minister Orban reiterated his warning to the rest of Europe, saying that “the fate of Christians in the Middle East should bring home to Europe that what is happening over there may also happen to us.” Hungary and other eastern European states have willingly accepted Christian refugees from the Middle East in the past.