The United States said Monday Egypt's military appeared to be clinging to power after ruling generals declared sweeping new powers just after a pivotal presidential vote.

"This is a critical moment in Egypt and the world is watching closely," State Department spokeswoman Victoria Nuland told a news conference.

"We are particularly concerned about decisions that appear to prolong the military's hold on power."

The State Department and Pentagon urged the Supreme Council of Armed Forces (SCAF) to make good on pledges for a swift transfer of power to a civilian government.

Washington's appeal came a day after the ruling military council announced de facto martial law, giving the armed forces control over the legislature and state budget while granting the generals veto power on a new constitution to be written by a hand-picked panel.

The military's move followed a ruling on Thursday by Egypt's constitutional court, which said a third of the parliament had been elected illegally and ordered the entire body dissolved.

"We call on the Supreme Council of the Armed Forces to restore popular and international confidence in the democratic transition process, by following through on their stated commitments to an inclusive constitutional drafting process, the timely seating of a democratically elected parliament and the swift permanent transfer of power to a civilian government," Nuland said.

Citing previous comments by Secretary of State Hillary Clinton, she said "there can be no going back on the democratic transition."

Asked if Washington would withhold military aid to Cairo, she said that "the decisions that are taken in this crucial period are naturally going to have an effect on the nature of our engagement with the government and the SCAF moving forward."

The United States expected the ruling military in Egypt to hand over "full power" to elected civilian leaders as it has promised, Pentagon spokesman George Little told reporters

"We have and will continue to urge the SCAF to relinquish power to civilian elected authorities," he said.

As polls closed on Sunday evening in Egypt, the ruling military issued a new constitutional document replacing an original declaration issued in March 2011 after an uprising that ousted the regime of Hosni Mubarak.

The document effectively stripped authority from a new civilian government.

The US military, which forged strong ties to the Egyptian armed forces over decades, was clearly concerned about the political situation.

"We believe Egypt's transition must continue and that Egypt is made stronger and more stable by a successful transition to democracy," Little said.

"Egypt has an enduring role as a security partner and leader in promoting regional stability and we look forward to working with the new government on a host of issues."

Little added that the ruling military council "is fully aware" of US concerns.

The Pentagon remained "hopeful that the right course will be adopted," said spokesman Captain John Kirby.

Muslim Brotherhood candidate Mohammed Mursi claimed victory on Monday in Egypt's vote, the country's first post-Mubarak presidential election.

Despite the military's assertion of authority, the Muslim Brotherhood's political arm on Monday insisted Egypt's parliament still had the power to legislate.

Amid political turmoil in Cairo, the US government also expressed concern over the security situation in the Sinai peninsula after an attack on Israel Monday by militants who crossed the Egyptian border.

An Israeli civilian was killed in the assault which triggered a firefight with the Israeli army that left at least two gunmen dead.

The United States condemned the attack and said it remained "concerned about the security situation in the Sinai peninsula and call for restraint on all sides," Nuland said.

"More broadly, we encourage the Egyptian government to find a lasting resolution to the issue of Sinai security."

Since Mubarak stepped down in February 2011, Sinai has been swept by unrest, fueling fears in Israel, which shares a 240-kilometer (150-mile) border on the Egyptian peninsula.