Two spokesmen for Egyptian president Mohamed Morsi have resigned in the latest defections from his administration in the face of a nationwide protest against his Islamist rule.

Career diplomats Omar Amer and Ihab Fahmy have stepped down after five months of speaking on behalf of Mohamed Morsi. The move came after six cabinet ministers quit and the army gave the president a 48-hour ultimatum to "meet the people's demands".

The army warned that differences had to be settled or it would impose its own solution on the country - despite ruling out getting involved in policymaking.

The ultimatum expires Wednesday but Morsi has remained defiant. He said: "[Egypt] will not accept under any circumstances to move backward.

"The people of Egypt have sacrificed the blood of their children, stability and development to build their new state, and we all choose democratic means as the only solution and peaceful way to manage our differences," he said.

US president Barack Obama urged Morsi to respond to opposition demands.

"President Obama encouraged President Morsi to take steps to show that he is responsive to their concerns, and underscored that the crisis can only be resolved through a political process," a White House statement said.

"Democracy is about more than elections. It is also about ensuring that the voices of all Egyptians are heard and represented by their government, including the many Egyptians demonstrating throughout the country."