"Now means now," White House spokesperson Robert Gibbs said of Egypt's transition, repeating that continued aid to Egypt would be influenced by the government's response to the crisis.
With its allies across the Arab world shaken by popular anger, the call for an immediate end to three decades of authoritarian rule in Egypt coincided with American hopes that reforms in Jordan and Yemen could stave off similar revolt.
It represented something of a dual approach for the Obama administration, which has gradually shed its support for the 82-year-old Mubarak while looking to shore up its other Arab friends facing much of the same resentment if not yet imminent revolution.
"We want to see free, fair and credible elections," State Department spokesperson PJ Crowley said on Wednesday. He stressed that keeping presidential elections in September or advancing the timeframe was entirely up to Egyptians and that the process should not be so hasty that it leaves legitimate players out of the process, but he added, "the sooner that can happen, the better".
Outrageous and deplorable
A day after President Barack Obama pressed Mubarak to loosen his three-decade grip on power immediately, clashes between protesters and pro-government supporters further alienated Egypt's besieged government from its longtime patron, the United States.
The administration decried the fighting that started when several thousand Mubarak supporters, including some riding horses and camels and wielding whips, attacked anti-government protesters.
Demonstrators dragged some of the attackers to the ground and beat them bloody, and the two sides rained stones and bottles down on each other.
Crowley called the assailants "thugs". Obama "found the images outrageous and deplorable", Gibbs said. The administration did not accuse Mubarak of orchestrating the clashes but said his government should show restraint.
"If any of the violence is instigated by the government it should stop immediately," Gibbs added.
Protesters claimed plainclothes police were among the attackers.
The comments from the two US spokesperson aimed to keep the pressure on Mubarak amid fears that the Egyptian government was trying to outlast the protesters' calls for democratic change with cosmetic changes that don't meet the need for real reform. They echoed Obama's call for change to "begin now" after Mubarak announced he would not run for re-election.
Meanwhile, Secretary of State Hillary Rodham Clinton demanded in a telephone conversation with Egypt's new vice president, Omar Suleiman, that the government investigate the events and bring those responsible to account. She condemned the violence, Crowley reported.
"We don't know who unleashed these thugs on the streets of Cairo," he said, but called it a clear attempt to silence Egyptian voices of dissent. "The use of violence to intimidate the Egyptian people must stop."
The day of strife in Egypt underscored the unsure post-Mubarak future for the country. His declaration not to run again was accompanied by plans to shepherd the political changeover from his 30-year reign.
But hundreds of thousands of Egyptians continued to demonstrate, determined to chase him from office.
Despite coming down increasingly harder in favour of the protesters, the US was still keen to promote an ordered transition to safeguard Egypt's status as a powerful purveyor of American influence in the Middle East from Arab-Israeli peace talks to countering Iran and fighting terrorism.
Still, the administration would not join the chorus calling for Mubarak's prompt resignation, offering a shred of support for a leader who has long been loyal to the United States.
In contrast to the sharp tone on Egypt, the administration cautiously praised reforms in Yemen by another pro-Western president who pledged not to stay on beyond his current term in an attempt to head off his country's version of the pan-Arab unrest sparked last month by the Tunisian protesters who overthrew their president.
Crowley welcomed President Ali Abdullah Saleh's "positive statements" about including opposition elements in a reform process after three decades in control of his country, which has become a main battleground against al-Qaeda.
Saleh is seen as a weak but increasingly important partner of the United States, allowing American drone strikes on al-Qaeda targets and stepping up counterterrorism co-operation.
The United States also was keeping a close watch on developments in Jordan, the only Arab country beside Egypt to have concluded a peace agreement with Israel, where the powerful Muslim opposition urged King Abdullah II's newly appointed prime minister to step down.