"Surely, there will be difficult days to come, but the United States will continue to stand up for the rights of the Egyptian people and work with their government in pursuit of a future that is more just, more free and more hopeful," Obama told reporters in the State Dining Room after speaking with the long-time leader from the White House.

The president made his comments on television shortly after he and Mubarak spoke. The half-hour phone call was initiated by the White House.

The conversation between the two leaders followed closely on a middle-of-the-night TV speech in which Mubarak, in Cairo, announced he was sacking his government to form a new one that would accelerate reforms. At the same time, he said, violence by protesters would not be tolerated.

Obama's remarks capped a day in which his administration struggled to keep abreast of developments in Egypt, where Mubarak ordered police and then the military into the streets in response to the thousands of protesters.

Before Obama spoke, White House press secretary Robert Gibbs announced the administration might cut the $1.5 billion in annual foreign aid sent to Egypt, depending on Mubarak's response to the demonstrations.

Obama also repeated demands by Secretary of State Hillary Rodham Clinton for Egypt's government to restore access to the Internet and social media sites, cut by the authorities in an apparent attempt to limit the flow of information about the protests demanding an end to Mubarak's rule.

Obama noted the United States and Egypt have a close partnership, a reference to Mubarak's support over the years for peace with Israel.

But he said, "We've also been clear that there must be reform, political, social and economic reforms that meet the aspirations of the Egyptian people."

"When President Mubarak addressed the Egyptian people tonight, he pledged a better democracy and greater economic opportunity. I just spoke to him after his speech, and I told him he has a responsibility to give meaning to those words; to take concrete steps and actions that deliver on that promise," Obama said. "Violence will not address the grievances of the Egyptian people, and suppressing ideas never succeeds in making them go away."

He added that the demonstrators had a responsibility "to express themselves peacefully. Violence and destruction will not lead to the reforms they seek."

Obama's decision to speak about the crisis in Egypt underscored the enormous U.S. interest at stake — from Israel's security to the importance of the Suez Canal and the safety of thousands of Americans who live and work in Egypt.

Gibbs said Obama had been briefed repeatedly during the day about the events unfolding half a world away.

The State Department issued a warning for Americans to defer all non-essential travel to Egypt.

Clinton said Mubarak should seize the moment to enact the long-called-for economic, political and social reforms that the protesters want. She said authorities must respect the rights of the Egyptian people to freedom of speech, assembly and expression.

"We are deeply concerned about the use of violence by Egyptian police and security forces against protesters, and we call on the Egyptian government to do everything in its power to restrain the security forces," Clinton said.

She sidestepped a question on whether the United States believed Mubarak was finished, but she said the U.S. wanted to work as a partner with the country's people and government to help realize reform in a peaceful manner. That underscored concerns that extremist elements might seek to take advantage of a political vacuum left by a sudden change in leadership.

Asked about U.S. aid to Egypt, Gibbs said the review would include both military and civilian assistance. Since Egypt made peace with Israel in 1978, the U.S. has plowed billions into the country to help it modernize its armed forces, and to strengthen regional security and stability. The U.S. has provided Egypt with F-16 jet fighters, as well as tanks, armored personnel carriers, Apache helicopters, anti-aircraft missile batteries, aerial surveillance aircraft and other equipment.

While the White House spokesman was emphatic in his calls for Mubarak and his government to abandon violence, he was less forceful on other issues.

Asked about Mohamed ElBaradei, a leading opposition figure who has been placed under house arrest, he said, "This is an individual who is a Nobel laureate" and has worked with Obama. "These are the type of actions that the government has a responsibility to change."

Like Clinton, Gibbs would not address Mubarak's future directly but said "we are watching a situation that obviously changes day to day and we will continue to watch and make preparations for a whole host of scenarios."

He also suggested contingency plans had been made for the evacuation of the U.S. Embassy in Cairo, should that become necessary.

Mubarak has long faced calls from U.S. presidents to loosen his grip on the country he has ruled for more than three decades since he replaced the assassinated President Anwar Sadat. Mubarak was Sadat's vice president and was slightly wounded in the attack in which Sadat died.

Mubarak has seen past U.S.-backed reforms in the region as a threat, wrote Ambassador Margaret Scobey in a May 19, 2009, memo to State Department officials in Washington.

"We have heard him lament the results of earlier U.S. efforts to encourage reform in the Islamic world. He can harken back to the Shah of Iran: the U.S. encouraged him to accept reforms, only to watch the country fall into the hands of revolutionary religious extremists," Scobey wrote in the memo, among those released recently by WikiLeaks. "Wherever he has seen these U.S. efforts, he can point to the chaos and loss of stability that ensued."

Senior lawmakers expressed growing unease with the developments, which could affect their deliberations on future assistance to Egypt.

Sen. John Kerry, a Democrat and chairman of the Senate Foreign Relations Committee, said Egypt's leaders must step back from the brink as Mubarak called in the military to help quell the protests that continued into the night, spreading in defiance of a curfew and attempts by police and security forces to break them up.

"In the final analysis, it is not with rubber bullets and water cannons that order will be restored," Kerry said. "President Mubarak has the opportunity to quell the unrest by guaranteeing that a free and open democratic process will be in place when the time comes to choose the country's next leader later this year."

Rep. Ileana Ros-Lehtinen, a Republican and chairman of the House Foreign Affairs Committee, said the protests were a sign that the Egyptian people's "cries for freedom can no longer be silenced." She said she was troubled by the "heavy-handed" government response.

"I am further concerned that certain extremist elements inside Egypt will manipulate the current situation for nefarious ends," she said.

Mubarak replaced the assassinated President Anwar Sadat. Mubarak was Sadat's vice president and was slightly wounded in the attack in which Sadat died.