Egypt’s military leaders on Monday sought to play down the significance of their move to sharply curtail the powers of the president, as U.S. officials said they were “deeply concerned” about the apparent power grab.

The generals’ attempted reassurances came amid growing indications that the Muslim Brotherhood’s candidate, Mohamed Morsi, had won the landmark presidential election, a victory that would make the Islamist group the military’s chief challenger for power. But Ahmed Shafiq, who served as the last prime minister under Hosni Mubarak, made a competing claim to have won, and members of the presidential election commission urged Egyptians to wait for official results, which are expected Thursday.

In a two-hour news conference, the ruling generals did not mention election results, and they did little to undercut the main message of the decree they had issued Sunday, just minutes after polls closed. The declaration left the armed forces virtually unaccountable to civilian rule and handed them legislative authority. It also gave the generals veto power over a body tasked with writing a new constitution, as well as total control over the military’s budget and the use of force.

State Department spokeswoman Victoria Nuland said the Obama administration would review all aspects of Egypt’s relationship with the United States, including military and economic aid, if the generals do not move quickly toward seating a president with full powers and allowing for the election of a new parliament.

“Decisions that are taken in this crucial period are naturally going to have an impact on the nature of our engagement with the government” and with the military leadership, Nuland said.

But the spokeswoman and others acknowledged uncertainty and confusion about the prevailing state of affairs and the seemingly contradictory military statements. “The concern is that the situation is extremely murky now; even many Egyptians don’t understand it,” Nuland said.

Although the United States has long been Egypt’s primary benefactor, experts said U.S. aid is among the least of the military’s concerns at the moment.

“They are fighting for what they see as their political survival . . . to prevent a different type of elite coming to power,” said Marina Ottaway, a senior associate at the Washington-based Carnegie Endowment for International Peace’s Middle East program. “What can the United States do to Egypt that essentially will make it worse for the military than having the Muslim Brotherhood in power?”

A ruling by Egypt’s constitutional court triggered the dissolution of the Islamist-dominated parliament last week. Although the generals have been widely seen as supporting that ruling, at the news conference, Maj. Gen. Mohammed el-Assar expressed regret over the move, saying that overseeing parliamentary elections had been the military council’s biggest achievement since it assumed power in February 2011.

“We were not happy with the dissolution of parliament,” Assar said. “But no one can comment on the rulings of the supreme Egyptian judiciary.” He added that although the generals had assumed legislative power until a new parliament is elected, in at least five months, the president would have the right to veto laws issued by the military council.

Some Islamists, liberals and others have challenged the military’s authority to dissolve parliament, and some Islamist legislators and independent lawmakers have vowed to convene as scheduled Tuesday. Legislators have been barred from entering the building, creating a potential for clashes.

The Muslim Brotherhood issued a statement Monday calling the military’s declaration a “coup” and urging the group’s followers to participate in protests against the dissolution of parliament and Sunday’s decree.

But at the news conference, Assar tried to assure Egyptians that the generals would manage the transition to democracy. “Let’s look ahead and not back. We all want what’s best for our country,” he said.

Some analysts said that, in exerting their authority, the generals might be gambling that Egyptians have been exhausted by 16 months of a tumultuous transition and will be unwilling to protest against them.

“This is about them approaching the end of the transition and worrying about their privileges and their power,” said Marc Lynch, a professor at George Washington University. “The fact that it is Islamists coming to power makes it easier to sell to the Egyptian public and to the West.”

Robert Springborg, an expert on the Egyptian military at the Naval Postgraduate School in California, said no one should have expected the generals to be subservient to a strong, elected civilian government. “The end goal has always been the same,” he said.

Despite the Brotherhood’s defiant tone toward the constitutional decree, Morsi was upbeat when he held an early-morning news conference declaring victory.

Morsi said he sought “stability, love and brotherhood for the Egyptian civil, national, democratic, constitutional and modern state” and made no mention of Islamic law.

Just after dawn Monday, Morsi supporters trickled into Cairo’s Tahrir Square to celebrate the conservative Islamist’s purported victory. Brotherhood’s predictions of election results have proven accurate in the past, and Morsi was ahead in the polls with 51.6 percent of the vote, according to preliminary results reported on the state-run al-Ahram Web site.

A statement from Shafiq’s campaign criticized the Brotherhood’s touting of unofficial results. Aides to Shafiq said their candidate was ahead by 250,000 votes late Monday, according to the state’s Middle East News Agency.

Staff writer Karen DeYoung in Washington contributed to this report.