White House Deputy spokesman Josh Earnest meanwhile said that Brennan called Hadi on Sunday to discuss the recent violence and the political transition leading up to the presidential election.

"Mr Brennan emphasized strongly the need for Yemeni security forces to show maximum restraint when dealing with demonstrations and called upon all sides to refrain from provocative acts that could spur further violence," Earnest said.

"Vice President Hadi said that he has initiated an investigation into the deaths and injuries that occurred and that he would do his utmost to prevent further bloodshed," Earnest said in Hawaii where Obama is on vacation.

Thirteen people were killed on Saturday in an attack by security forces and loyalists of Saleh against a march of thousands of people calling for the embattled leader's trial, medics said. Dozens more were injured.

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