The prospect of such a proxy war or a sectarian conflict along the lines of Lebanon is a major reason the United States is leery of arming the Syrian rebels or edging toward direct military involvement, though U.S. officials rarely say so.
Jordan has concerns about its territory, the Turks worry over Kurdish terrorists operating from inside Syria and neighboring Lebanon's is still struggling to emerge from decades of sectarian civil war and political instability.
Still, Moscow's calculus doesn't appear to be changing. Russia did begin inching away from Assad, particularly after the massacre in which dozens of women and children were among those shot at close range by pro-regime gunmen. And Foreign Minister Sergey Lavrov said this week that while both sides are to blame, Assad "bears the main responsibility for what is going on."
However, he also said the Houla massacre must not be a pretext to push for military intervention from outside. Instead, he urged all sides to focus on U.N. mediator Kofi Annan's peace plan, which by all accounts has failed to stem the violence.
Other than Russian opposition, there are other hurdles to armed intervention, U.S. officials said.
Clinton said Assad's foes lack the unity that the anti-Gadhafi camp eventually rallied in Libya. Syria's professional military and substantial air defenses also would make intervening far more difficult. And whereas in Libya the U.S. was able to count on the support of Gulf countries in monitoring a no-fly zone and carrying out some airstrikes, the Arab League is split on whether military options should be entertained in Syria.
"We know it could actually get much worse than it is," Clinton said. "We are trying to prevent that." Clinton underscored her support for the U.N. mediation effort, which she said had had some positive results even though none of its six points have been achieved. She stressed that U.N. observers have performed two important functions.
"In many of the areas where they are present, violence has gone down," Clinton said. "And they serve as independent observers, the eyes of the world if you will, in reporting back when terrible events like the recent massacre occur. They've tried to cut through the clutter and disinformation coming from the Syrian government."
She spoke as activists reported more shelling in the central Houla area.
The Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees activist groups said Thursday's shelling of Houla was mostly by heavy machine guns. Survivors of last week's massacre blamed pro-regime gunmen for the close-range shooting of civilians in their homes, though the government denied the involvement of its troops.
Carney said the U.S. can't deal with Syria by itself.
"There is no question that as mighty as the United States is, that we cannot end all atrocities around the globe," Carney said.
The United States must approach world problems with an eye to its own national security interests and to what is possible to achieve, Carney said, while "making sure you're not taking actions that create unintended consequences that are bad for the United States and bad, in some cases, for the very people you're trying to help."
Lee reported from Washington. Associated Press writers Anne Gearan in Washington and Jamal Halaby in Amman, Jordan, contributed to this report.