U.S. Vice President Joe Biden says Egyptian President Hosni Mubarak's resignation is a "pivotal moment" in history.
He spoke in the U.S. state of Kentucky, ahead of an expected statement in Washington by President Barack Obama.
Biden said the U.S. has said from the beginning of the political unrest that the future of Egypt will be determined by the Egyptian people.
He said the U.S. has stood for a set of core principles during the uprising, including deeming violence against demonstrators "unacceptable" and calling for the respect of Egyptians' rights. He said the U.S. has also urged that Egypt's political transition be an "irreversible change" and a negotiated path toward democracy.
Biden added that Democrats and Republicans have largely spoken with one voice on the issue, and said that unity will be even more important in the "delicate" days ahead.
The White House said President Obama was informed of Mr. Mubarak’s decision to step down during a meeting in the Oval Office. He then watched television coverage of the scene in Cairo for several minutes.
The Obama administration has repeatedly said that it is looking for an orderly transition that leads to free and fair elections.
In a statement late Thursday, Mr. Obama said the Egyptian government must put forward a credible, concrete, and unequivocal path toward genuine democracy.
The United States provides more than $1.3 billion in military aid to Egypt each year.
Some U.S. lawmakers had called for cutting that aid if the governmental transition in Egypt did not happen soon.