US Secretary of State Hillary Clinton has urged Iran and Russia to rethink their support for Syria.
Mrs Clinton said the worst scenarios of the conflict spilling beyond its borders could come to pass.
She said there are signs Iran is sending more people and increasingly sophisticated weaponry to support Syrian President Bashar al-Assad.
Rebels are seeking to end the Assad family's four-decades of authoritarian rule in a battle that has lasted 22 months.
Speaking on the eve of her State Department departure, Mrs Clinton also said Russia continues to send aid to the Syrian government, including financial help.
She voiced fears that the conflict, in which more than 60,000 people are believed to have died, may worsen internally and spread.
"I personally have been warning for quite some time of the dangers associated with an increasingly lethal civil war and a potential proxy war," Mrs Clinton told a group of reporters.
"Therefore, I think it's incumbent on those nations that have refused to be constructive players to reconsider their positions because the worst kind of predictions of what could happen internally and spilling over the borders of Syria are certainly within the realm of the possible now."
However, she declined to comment on reports that Israel had bombed Syria on Wednesday.
Diplomats, Syrian rebels and security sources said Israeli jets bombed a convoy near the Lebanese border on Wednesday, apparently hitting weapons destined for Hezbollah.
Syria denied the reports, saying the target was a military research centre northwest of Damascus and 13km from the border.
However, it warned of a possible "surprise" response to Israel over the reported attack.
Hezbollah, an Iranian ally that also supports Mr Assad, said Israel was trying to thwart Arab military power and vowed to stand by its ally.
Mrs Clinton said the US was worried that Iran had recently increased its support for Mr Assad.
She made similar comments about Russia.
"We have reason to believe that the Russians continue to supply financial and military assistance in the form of equipment," she said. "They are doing it in the recent past."
Russia has been Mr Assad's most important ally throughout the Syrian conflict, which began with peaceful street protests and evolved into an armed uprising against his rule.
Russia has blocked three Security Council resolutions aimed at pushing him out or pressuring him to end the bloodshed.
But Russia has also distanced itself from Mr Assad, by saying it is not trying to prop him up and will not offer him asylum.
Mrs Clinton appeared sceptical that Russian Prime Minister Dmitry Medvedev's comment this week that Mr Assad's chances of staying in power were growing "smaller and smaller" might herald a fundamental shift in Russia's stance.
She said all of the rhetoric over the past two years had failed to translate into changes in Russian policy.
Mrs Clinton praised the head of Syria's main opposition coalition, Mouaz Alkhatib, for saying this week that he was ready to hold talks with Assad representatives outside Syria if authorities released tens of thousands of detainees.
"I thought he was not only courageous but smart in saying that if certain conditions are met we will begin discussing a political transition because you have to you know make it clear that there will be something other than hardened fighters when this conflict finally ends," Mrs Clinton said.
"Otherwise, it might not ever end in the foreseeable future."