Hezbollah, which enjoys support from Syria and Iran, has been pressing Hariri to disavow a UN probe into the 2005 assassinaton of his father, former premier Rafiq Hariri. Press reports speculated that the Special Tribunal for Lebanon was on the verge of indicting senior Hezbollah members.
Secretary of State Hillary Clinton, on a visit to Qatar, denounced the "transparent effort by those forces inside Lebanon as well as interests outside Lebanon to subvert justice and undermine Lebanon's stability and progress."
"We believe that the work of the special tribunal must go forward, so justice can be served and impunity ended," she said.
"We believe that the leaders of Lebanon have an ongoing responsibility to serve the interests of their own people, not outside forces."
Analysts saw Hezbollah's move as an attempt to humiliate not just Hariri but also the United States, which considers the virulently anti-Israeli movement to be terrorists.
US officials said that the administration was reaching out to France, Saudi Arabia and Egypt to plan the next move on Lebanon. Saudi Arabia and Syria had been leading months of efforts to mediate the crisis.
Clinton will consult with regional players on Thursday when she takes part in the "Forum on the Future" conference in Qatar, State Department spokesman Philip Crowley said in Washington.
Crowley said it was too early to see if Hariri could put together a new government.
"Obviously there's no guarantees at this point," Crowley told reporters. "We will hope that no element within Lebanese society takes advantage of this or tries to incite violence in the aftermath of this."
The White House statement said Hariri and Obama "specifically discussed united efforts with France, Saudi Arabia, and other key international and regional actors to maintain calm in Lebanon and ensure that the work of the tribunal continues unimpeded by third parties."
Crowley said it was "hard to say" if Syria had a role in the government's collapse, pointing out: "Syria has a history of interfering in Lebanese affairs."
The Obama administration, despite opposition in Washington from the rival Republican Party, has appointed an ambassador to Damascus as part of its policy of reaching out to adversaries, but also sharply criticized Syrian policy.
The United States has stood firm in support of the tribunal into the Hariri assassination, a pivotal event that set off street protests in Beirut leading Syria to withdraw troops after nearly three decades in Lebanon.
"Hezbollah is presenting a false choice for Lebanon of justice or stability. We think that Lebanon deserves both," Crowley said.
But Graeme Bannerman, a Lebanon expert at the Washington-based Middle East Institute, said US insistence on the tribunal had backfired as it was clear many Shiites -- one of the country's three main groups -- would not accept it.
"Probably no one in the world wants the tribunal more than Hariri, but he has a whole series of other considerations. He can't govern without consensus" among the three groups, he said.
In backing the tribunal, the United States also has "an alternative agenda, which is to weaken Hezbollah and therefore Syria and Iran. It doesn't take into consideration how the Lebanese political system works."
"I think we are an essential part of Hariri's problem. I think we are making his life more difficult rather than easier."