The UN has expressed concern over a radical EU-Turkey plan to ease the migrant crisis, saying it could contravene international law.

Under the plan, all migrants arriving in Greece from Turkey would be returned and for each Syrian sent back, a Syrian in Turkey would be resettled in the EU.

The UN’s refugee agency said any collective expulsion of foreigners was “not consistent with European law”.

Amnesty International called the plan a death blow to the right to seek asylum.

The deal, discussed at a summit in Brussels on Monday, has not been finalised and talks will continue ahead of an EU meeting on 17-18 March.

Europe is facing its biggest refugee crisis since World War Two. Last year, more than a million people entered the EU illegally by boat, mainly going from Turkey to Greece.

Most of them were Syrian, fleeing the country’s four-year civil war. Another 2.7 million Syrian refugees are currently in Turkey.

What’s in the EU-Turkey proposal?

The EU heads said “bold moves” were needed to tackle the crisis, and made the following proposals:

• All new irregular migrants crossing from Turkey to the Greek islands will be returned to Turkey, with the EU meeting the costs. Irregular migrants means all those outside normal transit procedures, ie without documentation. The term “illegal migration” usually refers to people smuggling
• In exchange for every returned Syrian, one Syrian from Turkey will be resettled in the EU
• Plans to ease access to the EU for Turkish citizens will be speeded up, with a view to allowing visa-free travel by June 2016
• EU payment of €3bn ($3.3bn; £2.2bn) promised in October will be speeded up, and a decision will be made on additional funding to help Turkey deal with the crisis. Turkey reportedly asked for EU aid to be increased to €6bn
• Preparations will be made for a decision on the opening of new chapters in talks on EU membership for Turkey