A major aid agency says 27.8 million people around the world were internally displaced by conflict and natural disasters last year, or as many as the combined populations of New York City, London, Paris and Cairo.

A report by the Norwegian Refugee Council on Wednesday says 8.6 million of last year's internally displaced were uprooted by conflict, more than half of them in Syria, Yemen and Iraq.

The group says Yemen alone accounted for one quarter of conflict-related displacement worldwide last year, with 2.2 million people uprooted, or 20 times more than in 2014.

The NRC's Middle East director, Carsten Hansen, says that richer, more stable countries are trying to keep out asylum seekers and that "millions remain trapped in their own countries with death ... just around the corner."