WASHINGTON — The Obama administration sharpened its response to political upheaval and brutal crackdowns in Egypt on Wednesday, telling its closest ally in the Arab world it must respond to its people's yearnings for democracy as the largest political protests in years swept Cairo streets.

But with no clear picture emerging of a democratic and pro-Western alternative to the three-decade rule of Egypt's authoritarian President Hosni Mubarak, it was unclear how hard the United States was willing to press its case.

A day after delivering a measured response to Egypt's demonstrations, Secretary of State Hillary Rodham Clinton said Egypt had to adopt democratic and other reforms and allow peaceful protests. She told Cairo to lay off social media sites like Facebook and Twitter even as activists are using them to organize street gatherings and destabilize the government.

The White House declined a direct opportunity to affirm support for Mubarak, who traveled to Washington to meet President Barack Obama just four months ago. Asked if the administration still backed Mubarak, White House spokesman Robert Gibbs would say only: "Egypt is a strong ally."

The tougher tone came as the U.S. struggles to confront an explosion of instability in the Middle East as Arabs from Tunisia to Yemen rebel against decades of political repression. Adding to the confluence of crises is the emergence of an Iranian-backed militant movement as Lebanon's dominant force and potentially embarrassing revelations creating new obstacles to Israeli-Palestinian peace.

Clinton said Mubarak's government had the power to ease tensions with anti-government activists, who defied an official ban on protests Wednesday by pelting police with firebombs and rocks in a second day of clashes. Police forces used tear gas and fired live ammunition in the air to disperse demonstrators. Some people were beaten.

"I do think it's possible for there to be reforms and that is what we are urging and calling for," Clinton told reporters at a State Department news conference with visiting Jordanian Foreign Minister Nasser Judeh.

"It is something that I think everyone knows must be on the agenda of the government as they not just respond to the protests but as they look beyond as to what needs to be done."

The protests against Mubarak's three-decade grip on power were inspired by the ouster of another long-time leader, Tunisian President Zine El Abidine Ben Ali, in a popular uprising nearly two weeks ago.

The day before Ben Ali fled into exile in Saudi Arabia, Clinton delivered a stark warning to Arab leaders across the Middle East that they faced possible revolt if they failed to address rampant social problems, repression and corruption that have alienated their populations, particularly the educated youth. Foundations of development and progress in the region were "sinking into the sand," she warned.

But having spent billions of dollars supporting its few Arab friends for decades, the United States is nearly as large a target for the unrest as the authoritarian regimes under siege.

U.S. officials won't paint the problem as one of democracy versus loyalty, but Washington's labored approach to the protests in different countries illustrates a complicated blend of political idealism and realpolitik. It also points up the unpredictability of the tinderbox of Arab populism.

Egypt represents the greatest challenge because of its strategic position bridging two continents, leadership status in the Arab world, lasting peace with Israel and the possibility of a hardline, Islamist movement filling the vacuum were Mubarak to be deposed.

The United States has urged peaceful political change in Egypt for years, but has tolerated routine police, judicial and human rights abuses there. It has provided Egypt with tens of billions of dollars in aid since it made peace with Israel in 1978. Last year, the country got more than $1.5 billion in economic support and military assistance from the U.S.

Unlike Tunisia, a second-tier U.S. ally, Egypt has been the bulwark of American influence in the Middle East and served as an economically impoverished but politically powerful intermediary in the Israeli-Palestinian conflict and beyond. When Mubarak last visited the White House in September it was to help relaunch moribund peace talks between Israel and the Palestinians.

Waiting in the wings is the Muslim Brotherhood, a cross-border Arab movement that has presented itself as the main opposition to Mubarak's rule in Egypt. That prospect is frightening to the United States and other Western nations because of its opposition to Israeli-Palestinian peace and much of the U.S. agenda in the region. Mubarak's weakened health in an election year and the questionable support for hereditary transfer of power to his son Gamal underscore the unsure footing for the country.

Jordan, the only other Arab state to make peace with Israel, is similarly vital to U.S. interests. Standing beside Clinton at the news conference, Judeh downplayed the chances of protests like those Tunisia and Egypt erupting in his country. He allowed that Jordanians have vented over rising oil and food prices, but maintained his country has the political openness to allow debate and dissent.

That may not be the case in Yemen, a nation plagued by corruption, inequality and political divisions and which has emerged as a main battleground against al-Qaida. Yemen's government under the weak president Ali Abdallah Saleh fails most democratic litmus tests, but it has allowed U.S. drone strikes on suspected terrorists on its soil and become a key counterterror partner. It is unclear how instability and upheaval there would serve American interests.

Linked to all of these crises is the diminished power of the United States — from global economics, where high U.S. unemployment and debt contrasts with booming growth rates in China, to the Middle East, where an intractable Iran has extended its influence over a large swathe of the Arabian peninsula.

The U.S. has spent hundreds of millions promoting a pro-Western government in Lebanon, but saw the fragile coalition toppled by the Shiite militant group Hezbollah, which gets its backing from Iran. Hezbollah now holds the power in a new government being formed. That the process occurred constitutionally hasn't made it any easier for Washington to accept, and the Obama administration is threatening to withhold further direct support.

Clinton said Wednesday that Israeli-Palestinian talks remained the No. 1 priority for the region, but an elusive peace deal six decades in the making was further complicated by this week's release of papers alleging wide-ranging Palestinian concessions.

Clinton insisted that the U.S. remained "absolutely committed" to the peace process, though there's little to indicate that sentiment is shared by all parties.

A final deal remains the official U.S. goal for this year, but prospects for such a monumental achievement failed to warrant even a mention in President Barack Obama's State of the Union address Tuesday — a suggestion the administration isn't too hopeful, either.