More than 4,200 Iraqis from Mosul fled to Syria in May, the United Nations refugee agency said on Friday, adding that it is gearing up for up to 50,000 people to leave the ISIL-held city and head across the border.

“Picture this, we have refugees fleeing to Syria. So it must be desperate. Reasons are the impending battle to take it (Mosul)," UNHCR spokeswoman Melissa Fleming told a briefing.

"This in anticipation of 50,000. There are contingencies for potential numbers coming in," she added.