Obama to address violence in Egypt, Kerry calls clashes a 'serious blow'

President Obama plans to make a statement about the violence in Egypt amid clashes between the military-backed government and Muslim Brotherhood supporters that so far have left more than 500 dead.

The president is addressing the issue on Thursday from Martha's Vineyard, where he is on vacation with his family. His administration has urged both sides to step back as the country is engulfed by violence.

Secretary of State John Kerry made a public appeal to Egypt's leaders on Wednesday, saying the deadly clashes have dealt a "serious blow" to reconciliation efforts and calling for calm.

"This is a pivotal moment for all Egyptians," Kerry said, in a surprise appearance during the daily State Department briefing. "The path toward violence leads only to greater instability, economic disaster and suffering."

Kerry condemned the violence that killed at least 149 people nationwide, as well as the restoration of emergency rule. And he urged Egypt's interim leaders to take a step back and calm the situation.

The White House earlier condemned the violence, warning the military-backed government that the "world is watching" amid a series of assaults on protest vigils among supporters of ousted President Mohammed Morsi.

"Violence will only make it more difficult to move Egypt forward on a path to lasting stability and democracy and runs directly counter to the pledges by the interim government to pursue reconciliation," White House spokesman Josh Earnest said. "The world is watching what is happening in Cairo.

We urge the government of Egypt and all parties in Egypt to refrain from violence and resolve their differences peacefully."

In the latest development, Egyptian interim Vice President Mohamed ElBaradei resigned to protest the crackdown on pro-Morsi sit-in camps.

President Obama, in the midst of a weeklong vacation on Martha's Vineyard, was briefed on the deteriorating situation Wednesday morning, Earnest said.

The White House's response largely echoed its previous statements since the democratically-elected Morsi was ousted on July 3.

The U.S. has said it does not plan to label Morsi's ouster a coup, and officials on Wednesday indicated no change in that position following the latest outbreak of violence.

"We have determined that it is not in the best interest of the United States to make that determination," Earnest said.

At least 149 people were killed in Egypt Wednesday. Amid the violence, Egypt's interim president also declared a monthlong state of emergency, ordering the armed forces to support police in efforts to restore law and order.

Wednesday's assault came after days of warnings by the interim administration that replaced Morsi after he was ousted. The two sit-in camps at two major intersections on opposite sides of the Egyptian capital began in late June to show support for Morsi. Protesters -- many from Morsi's Muslim Brotherhood -- have demanded his reinstatement.

By law, the U.S. would have to cut off $1.3 billion in aid to Egypt if it determined that Morsi's ouster had been a coup. Officials say taking such a step would not be in the U.S. interests, leading the administration to conclude that it will not make a coup determination.

At least 250 people have died in previous clashes since the coup.

The Associated Press contributed to this report.