The top US defense official expressed "concern" about recent developments in Egypt in a call on Sunday to Egyptian army chief General Abdel Fattah al-Sisi, the Pentagon said.
Defense Secretary Chuck Hagel expressed his condolences for the victims of a spate of recent bomb attacks in Egypt, and offered US assistance to investigate the incidents, a Pentagon spokesman said in a statement.
A bomb exploded outside an Egyptian army building north of Cairo on Sunday, the latest in a series of violent incidents in Egypt.
The Egyptian Army labeled the incident a terrorist attack, but did not name the Muslim Brotherhood, the Islamist group it declared a terrorist organization last week.
In his call with Sisi, Hagel also "stressed the role of political inclusiveness," and the two men discussed "the balance between security and freedom," spokesman Rear Admiral John Kirby said in the statement.
"Secretary Hagel also expressed concerns about the political climate in advance of the constitutional referendum, including the continued enforcement of a restrictive demonstrations law," Rear Admiral John Kirby said.
Egypt's army-backed government has used the new classification to detained hundreds of supporters of the Muslim Brotherhood, and thousands more are already in jail.
The terrorist classification was the government's latest move to crack down on the Islamist group following the ouster of former President Mohamed Mursi in July.
As friction grows between supporters and opponents of the Brotherhood, officials have also warned Egyptians against participating in protests in support of the group. Street clashes have killed seven people in the last three days.
The Brotherhood, which has estimated its membership at up to a million people, was Egypt's best organized political force until this summer's crackdown. A political and social movement founded in 1928, it won five elections after the downfall of President Hosni Mubarak in 2011.
Under the government's political transition plan, a referendum is planned for mid-January on a new constitution, followed by parliamentary polls and a presidential election.