Marines at bases in Spain and Italy were poised Tuesday for the possibility of rushing to Cairo to protect or evacuate U.S. citizens and the U.S. Embassy amid the ongoing violence and political unrest in Egypt.

"We do believe we have taken steps to ensure our military is ready to respond to a range of contingencies" flowing from the faceoff between President Mohammed Morsi and opposition protesters that has the Egyptian military threatening to intervene, said George Little, the Pentagon’s chief spokesman.

In response to the killing last year of Ambassador Chris Stevens in Benghazi, Libya, the U.S. posted 550 Marines to the air base in Moron, Spain, to serve as a response force to turmoil in the region.

As the Egyptian protests have escalated, elements from the Marine force at Moron have been moved closer to Cairo at the air base at Sigonella, Italy, Pentagon officials said. The Marines have MV-22 Osprey tilt-rotor aircraft available at both Moron and Sigonella to move them quickly to any contingency.

Last Thursday, and again on Monday, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, phoned Egyptian Gen. Sedki Sobhi to gain assurances on the safety of U.S. citizens in Egypt, Pentagon officials said. Morsi picked Sobhi to become Egyptian chief of staff after dismissing his predecessor, Lt. Gen. Sami Hafez Anan.

Dempsey’s phone call to Sobhi on Monday came on the same day that the Egyptian military threatened to take over the government in 48 hours if Morsi and the opposition failed to resolve their differences.

However, hundreds of thousands of protesters calling on Morsi to step down remained in Cairo’s Tahrir Square, birthplace of the 2011 uprising against former President Hosni Mubarak.

Across town, Morsi's backers from the Muslim Brotherhood also remained at their own rally site, where they vowed to resist a military coup.

Gehad al-Haddad, a senior Muslim Brotherhood spokesman, told CBS News that the military’s threat of a takeover was serious.

"I think it's a coup," al-Haddad said.

On Monday night, President Obama, who was wrapping up his week-long visit to Africa, phoned Morsi "to convey his concerns about recent developments in Egypt" and to urge Morsi to be responsive to the demands of the protesters, the White House said.

Obama told Morsi that democracy means "ensuring that the voices of all Egyptians are heard and represented by their government, including the many Egyptians demonstrating throughout the country," the White House said.

Obama also "stressed his expectation that the government of Egypt continue to protect U.S. diplomatic personnel and facilities," the White House said.