The United States on Wednesday voiced concern that a call by Egypt's army chief for rallies in his support could trigger more violence and instability in the country.

General Abdel Fattah El-Sisi urged people to come out onto the streets on Friday to "give me a mandate and command to end terrorism and violence."

But Washington said any demonstrations should be peaceful and that Egypt's interim leaders, in place since the toppling of president Mohamed Morsi on July 3, must remain focused on moving towards elections.

"We're very concerned about the calls," State Department spokeswoman Jen Psaki said.

"We're concerned about the possibility of this leading to more violence."

She added that "clashes would make it very difficult to reconcile and get ahead of the cycles of unrest and instability."

She insisted the United States remained focused on "encouraging the interim government to move towards an inclusive process, which includes elections."

"We're monitoring closely steps they're taking to do just that," she said.

A coalition of Islamists led by Morsi's Muslim Brotherhood meanwhile said they would press ahead with their own rallies on Friday.

"Sisi's threats are an announcement of civil war," the group said.

Meanwhile, President Barack Obama has decided to halt the delivery of F-16 fighter jets to Egypt for an undetermined period due to the "current situation" on the ground there, the Pentagon said on Wednesday.

"Given the current situation in Egypt, we do not believe it is appropriate to move forward at this time with the delivery of F-16s," Pentagon spokesman George Little told reporters, adding Obama's decision was made with the unanimous consent of his entire national security team.