The window of opportunity for a peaceful solution to the ongoing crisis in Syria is closing, a top U.S. official said on Thursday, adding that the international community had to unite against the regime of Syrian President Bashar Assad.

Earlier Thursday, in another reference to escalating violence in Syria, U.S. Secretary of State Hillary Clinton rejected the possibility of a military intervention in the country, saying

"A lot of people are trying to figure out what could be an effective intervention that wouldn´t cause more death and suffering," Clinton said, arguing Syria´s population density increased the odds of civilian casualties in any armed action.

However, speaking on the subject later in the day, White House spokesman Jay Carney said that U.S. President Barack Obama "and others have made clear that the window of opportunity here to allow for a peaceful political transition in Syria is -- will not remain open for long."

"There is an urgent need for the United -- the international community to come together and further unify against the Assad regime in an effort to persuade the Assad regime and pressure and isolate the Assad regime to the point where that transition is allowed to fully take place," Carney added.

Carney added that if that solution is not reached "the consequences are very serious."

"And that´s what Ambassador Rice was talking about, and Secretary Clinton and I, because the consequences of not taking that firm action are more violence -- violence that spills over Syria´s borders; violence that results in even greater participation in this by Iran, for example, and others, to the point where it becomes a proxy war of sorts," he added, saying: "And this is bad for the region and bad for the Syrian people and bad for the world."

Also referring to the possibility of a peaceful resolution of the Syrian crisis, U.S. State Department Deputy spokesman Mark Toner said on Thursday that "Assad, his regime is the greatest stumbling block right now."

"They´ve failed to comply with any of the six components of the Annan plan. They´ve continued to besiege population centers, including the horrible events that happened in Houla over the weekend. You know, let´s put responsibility for this bloodletting squarely upon Assad," Toner added.

Also on Thursday, U.S. Ambassador to the U.N. Susan Rice responded to reports on a Russian arms ship that had docked at the Tartous harbor in Syria.

"With respect to the reported docking of a ship carrying Russian arms, this is obviously of the utmost concern given that the Syrian government continues to use deadly force against civilians, she added, saying: "It is not technically, obviously, a violation of international law since there’s not an arms embargo, but it’s reprehensible that arms would continue to flow to a regime that is using such horrific and disproportionate force against its own people."

Friday´s massacre of more than 100 civilians in Houla, many of them children, has triggered calls for the West to take more robust action in Syria, despite Russian and Chinese opposition.

Clinton said earlier Thursday that she had not given up on the possibility of persuading Russia to support stronger action against the Assad government, saying she had made the case that the chances of a full-blown civil war were higher if the world failed to act.

"The dangers we face are terrible," she said, saying the violence between government forces and pro-Assad militias against the opposition forces would turn into something much worse.

"[That] could morph into a civil war in a country that would be driven by sectarian divides, which could then morph into a proxy war in the region because remember you have Iran deeply embedded in Syria," she said. (© Copyright 2012 Ha´aretz 06/01/12)