Landmines are causing 50-70 casualties a week - about the number that usually hit all of Afghanistan in a year

BEIRUT, Feb 6 (Reuters) - Unexploded landmines left by Islamic State fighters cover every inch of their former Syrian capital of Raqqa, a U.N. official said on Tuesday, advising civilians against returning to their homes there.

About 60,000 Syrians have returned to the ruins of Raqqa since Islamic State left, Panos Moumtzis, U.N. assistant secretary general and regional humanitarian coordinator for the Syria Crisis, told reporters in Beirut.

"The number of unexploded ordnance in Raqqa is something that we have never seen before. Extreme. Every house, every room, every inch of the city," he said.

Landmines are causing 50-70 casualties a week - about the number that usually hit all of Afghanistan in a year - he said.

Though the United Nations was not helping people to go back to Raqqa, it could not stop people returning spontaneously to homes which could harbour danger, he said.

An alliance of Kurdish and Arab militias backed by a U.S.-led coalition captured Raqqa in November after a long campaign to besiege the city followed by months of fighting inside it.

As well as landmines and improvised explosives planted by Islamic State, there may also be undetonated bombs and missiles used by the coalition in its massive blitz of Raqqa.

The jihadist group still holds a few scattered pockets in Syria and Iraq, but has lost almost all its possessions in the two countries, where at its height it ruled over millions of people.

As when attacked in other parts of its self-declared "caliphate", it used many booby traps as a tactic against its advancing foes. But in Raqqa many of the hidden bombs appear aimed at civilians.

"It is vicious to think they are deliberately planted to kill, to kill civilians, to kill people," said Moumtzis.

(Reporting By Angus McDowall; Editing by Richard Balmforth)

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