Details added (first version published at 03:57)US President
The US leader spoke from the White House, shortly after telephoning with Mubarak, who had just given his own address vowing not to step down but announcing that he was firing his cabinet.
"What's needed right now are concrete steps that address the demands of the Egyptian people," Obama said.
Earlier Friday, the United States threatened to withhold aid to Egypt if Mubarak's regime does not immediately halt violence against protesters and move toward introducing reforms.
Obama said he told Mubarak that he needed to follow through on his promise of reforms.
"He has a responsibility to give meaning to those words," Obama said.
He stressed the United States' chief concern was the prevention of violence, calling on both the government and protesters to act peacefully. He called on the Mubarak regime to restore internet access and blockages of mobile phones that have hampered communications.
The US has been a long-time ally of Mubarak, but Obama stressed in his remarks that Washington stands behind the universal rights of the Egyptian people.
"The people of Egypt have rights that are universal," he said. "That includes the right to peaceful assembly and association, the right to free speech, and the ability to determine their own destiny. These are human rights. And the United States will stand up for them everywhere."
He said the protests were a "moment of volatility that has to be turned into a moment of promise."
"The United States has a close partnership with Egypt. And we've cooperated on many issues, including working together to advance a more peaceful region," he said. "But we've always been clear that there must be reform: political, social and economic reforms that meet the aspirations of the Egyptian people."
White House spokesman Robert Gibbs said the US will be reviewing all forms of aid to the Egyptian government. The US provides more than 1.5 billion dollars annually in aid to Egypt, most of it military assistance.
"We are watching very closely the actions of the government, of the police, of all the security forces, and all of those in the military," Gibbs said. "That their actions may affect our assistance would be the subject of that review."
"We've been very clear about what needs to happen: Violence in any form should stop immediately and grievances should be addressed," Gibbs said when asked what steps the Egyptian government must take to avoid a disruption of US assistance.
"We will monitor what is and what has happened, and future events, as we undertake a review of our assistance posture."
Mass protests started Tuesday. The protesters want an end to the 30-year reign of Mubarak, a close US ally and partner in the Arab- Israeli peace process.
The US has sought to back the rights of the protesters to express their views peacefully but without alienating Mubarak. The announcement by Gibbs, however, signalled that President Barack Obama is willing to increase pressure on Mubarak to accept change.
Secretary of State Hillary Rodham Clinton called Friday on the Egyptian government to "immediately engage" in dialogue to address the "deep grievances" of its people.
"As a partner, we strongly believe that the Egyptian government needs to engage immediately with the Egyptian people in implementing needed economic, political and social reforms," Clinton said, adding that "violence will not make these grievances go away."
"We continue to monitor the situation very closely. We are deeply concerned about the use of violence by Egyptian police and security forces against protesters," she said. "And we call on the government to do everything in its power to restrain security forces."
The White House and Clinton insisted that the Egyptian government restore communications in the country following reports that internet and access to social media websites like Facebook and Twitter were cut. The Egyptian authorities should "reverse the unprecedented step ... taken to cut off communications," Clinton said.
Gibbs said the daily national security briefing Obama received Friday was entirely devoted to the situation in Egypt. The possibility of cutting aid was among the scenarios discussed, Gibbs said.
Meanwhile, the State Department urged Americans on Friday to avoid travelling to Egypt because of dangers posed by the unrest, and Delta Air Lines announced Friday it will suspend all Cairo flights "indefinitely," starting Saturday.