The European Union set a June deadline for Poland and Hungary to start admitting their share of migrants from overstretched Italy and Greece or risk sanctions.

Eastern European countries like Hungary and Poland have opposed an EU plan adopted in 2015 to take in 160,000 Syrian, Eritrean and Iraq asylum seekers from Greece and Italy.

"I call on Poland and Hungary which have not relocated a single person ... to start doing so right now," EU Migration Commissioner Dimitris Avramopoulos told reporters in Strasbourg, France.

"If no action is taken by them before (our) next report in June, the Commission will not hesitate to use its powers under the treaties and open infringement procedures," he said.

Avramopoulos said 18,418 asylum seekers have now been relocated to other EU countries from the two Mediterranean states, saying it was a "last warning" for laggards to do their share.

Poland's interior minister played down fresh warnings of European Union sanctions over Warsaw's refusal to accept migrants, saying accepting them would have been "worse" than the EU rebuke.

Mariusz Blaszczak reacted Wednesday to fresh warnings by EU Migration Commissioner Avramopoulos for nations that do not take any migrants by June. Avramopoulos singled out Poland and Hungary.

Blaszczak argued on state radio that the "security of Poland and of the Poles" was at stake and drew a link to terror attacks in Western Europe. He added that accepting migrants would have "certainly been worse for Poland" than facing EU action.

On taking power in 2015, Poland's government reversed a decision by its predecessors to take in some 10,000 refugees.

Under so-called EU "infringement proceedings," Brussels sends a letter to national governments to demand legal explanations over certain issues before possibly referring them to the European Court of Justice. EU states can eventually face stiff financial penalties if they fail to comply.

European Commission chief Jean-Claude Juncker and other EU officials have long expressed frustration with the slow pace of relocation of asylum seekers aimed at helping to tackle the worst European migration crisis since World War II. More than one million migrants entered Italy and Greece in 2015.

European sources blamed the delays on a series of factors: governments trying to screen jihadists in the wake of terror attacks, a lack of housing and education for asylum seekers, and logistical problems.

They said some countries were setting unacceptable conditions by refusing Muslims, black people or large families, with Eastern European states the worst for discriminating on religious or racial grounds.

Brussels launched the relocation plan to help people fleeing the mainly Muslim war-torn countries of Syria and Iraq as well as the east African state of Eritrea.

Austria, which had asked for a temporary exemption to the scheme, has pledged to relocate 50 people from Italy. The Commission welcomed the move and urged Vienna to do the same from Greece.

The Commission also put pressure on the Czech Republic to resume relocations after having said it failed to take in people for nearly a year. It also urged Bulgaria and Slovakia to "show more flexibility" on the kinds of people they admit and said Ireland and Estonia should work to clear security concerns with Italy over admitting asylum seekers. The Commission called on Spain, Belgium, Croatia, Germany, Romania, Slovakia, France and Cyprus to raise the numbers they take in.