US Defense Secretary Chuck Hagel on Sunday called for a "transparent" constitutional referendum in Egypt to ensure all voters cast their vote "freely" on Tuesday and Wednesday.

In his talks over the phone with his Egyptian counterpart General Abdel-Fattah al-Sisi, the Pentagon chief discussed "the importance of the constitutional referendum for Egypt's overall political transition," Pentagon spokesman John Kirby said in a readout of the conversation.

Hagel "stressed the importance of a transparent referendum in which all Egyptians have the opportunity to cast their vote freely, " and urged al-Sisi to ensure the international and domestic observers have "full access," Kirby said.

Egypt has been hit with waves after waves of protests and a deteriorating security situation since Mohamed Morsi, the country's first elected president, was ousted by the military in early July, 2013.

Last month, the interim government declared the Muslim Brotherhood, to which Morsi belongs, a "terrorist group".