Geneva (AFP) - The United Nations on Tuesday urged states not to "backtrack" on pledges made to host migrants and refugees, including from Syria, in the wake of the attacks in Paris.

Multiple UN agencies issued a warning to European leaders who might use the tragedy to advance anti-migrant policies, stressing that many of those seeking refuge in Europe have also been the victims of extremists.

Of the estimated 1.1 million migrants who have landed in Europe via the Mediterranean since 2013, less than six "have raised questions in terms of possible links to extremism," said International Organization for Migration spokesman Joel Millman.

Millman noted that the IOM, which received that information from individual governments, has no evidence that any of those people were ever involved in acts of violence.

French sources have said that a passport found near the body of one of the Stade de France suicide bombers may have belonged to a Syrian -- possibly an ex-government soldier -- who registered on the Greek island of Leros on October 3.

Using that information as a pretext to curb migration by vulnerable people into Europe could prove disastrous, UN representatives said.

"We are concerned about the reactions from some states to end programmes being put in place, backtracking from commitments made to manage the refugee crisis," said UN refugee agency (UNHCR) spokeswoman Melissa Fleming.

"Refugees should not be turned into scapegoats and must not become the secondary victims of these most tragic events," Fleming added.

- 'Pretty disgusting' -

Earlier this year, European leaders agreed to share 160,000 migrants around the continent via quotas.

The scheme is still in its infancy, with the actual number of people who have been resettled so far estimated at below 1,000, Fleming said.

But some European populist and far-right leaders have seized on the jihadist attacks in Paris to demand the continent stem the migrant influx.

Hungary's parliament on Tuesday gave Prime Minister Viktor Orban the green light to challenge the EU quota system in court.

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France's anti-immigration National Front leader Marine Le Pen called for an "immediate halt" to new arrivals, while Germany's xenophobic PEGIDA movement drew thousands to their latest anti-Islam rally.

Poland's new right-wing, eurosceptic government, which was sworn in Monday, signalled it would take a harder line on migrants, while honouring previously agreed EU commitments.

"What's being threatened is an EU plan to manage the arrival of so many people... We need all the countries in Europe to be on board," Fleming said.

A spokesman for the UN rights agency, Rupert Colville, described calls for a crackdown on migrants as "very, very stupid."

"It's really pretty disgusting how extreme anti-immigrant politicians and media commentators have jumped on this attack to push their existing agendas. That is really pretty obscene actually, given what happened in Paris," Colville told reporters.