Mitt Romney charged vice president Joe Biden Friday with "doubling down on denial" as the White House struggled to combat a growing storm over the attack on the US consulate in Benghazi.

The latest exchanges battered an administration repeatedly thrown onto the defensive by the political reverberations of the attack on September 11.

White House spokesman Jay Carney was forced to clarify remarks by Biden which appeared to contradict evidence that US officials refused extra security for US posts in Libya prior to the Benghazi assault.

"Biden was speaking about himself and the president. He wasn't talking (about) the administration at large," Carney said.

Biden had said: "We weren't told they wanted more security".