In talks with Egyptian Defence Minister General Abdel-Fattah El-Sisi, US Defence Secretary Chuck Hagel has underlined the importance of a forthcoming constitutional referendum for the country's political transition after three years of turmoil, a pentagon statement said on Sunday.

The defence leaders discussed the importance of a "transparent referendum in which all Egyptians have the opportunity to cast their vote freely," John Kirby, Pentagon press secretary, said.

The 14-15 January vote is billed as the first milestone in a transitional roadmap set out by the interim authorities which will lay the groundwork for presidential and parliamentary elections by the summer. The authorities have yet to decide which vote will take place first.

An amended charter was finalised in December after the 2012 constitution was suspended following the army's ouster of Islamist president Mohamed Morsi after millions protested against his turbulent year in office.

Hagel also urged his Egyptian counterpart "to ensure that international and domestic observers have full access to the proceedings" of the poll, the press secretary added.

"Hagel and El-Sisi reaffirmed their commitment to strong US-Egyptian relations," Kirby said, "and the secretary wished General El-Sisi a blessed Mawlid Al-Nabi, an Islamic holiday."

Monday, an official holiday in Egypt, marks the birth anniversary of the Prophet Mohamed.

State news agency MENA quoted El-Sisi on Saturday as urging Egyptians to "turn out in force" to vote in the constitutional referendum in order to "correct the democratic path and build a modern democratic state."

El-Sisi on Saturday hinted that he could stand for president if there was "a request from the people and a mandate from my army."

The military has said 160,000 soldiers will be deployed to secure some 30,000 polling stations during the referendum in which around 53 million Egyptians are eligible to cast their ballots.

The vote will be Egypt's third constitutional referendum in three years, and its sixth poll since the 2011 popular revolt drove president Hosni Mubarak from power.