Hillary Clinton, the outgoing US secretary of state, has warned that Iran is stepping up its support for Syrian President Bashar al-Assad while Russia is continuing to send money and arms to the regime.
Clinton on Thursday accused Tehran of sending into Syria "more personnel" to support and advise military security forces in their fight against the armed opposition.
In her last media interview as secretary of state, Clinton said there was "a lot of concern that they are increasing the quality of weapons, because Assad is using up the weapons, so it's numbers and it's material".
"The Iranians have made it clear for some time that keeping Assad in power was one of their highest priorities," she told a roundtable of journalists.
"The Iranians have been actively involved from the very beginning. It appears that they may be increasing that involvement and that is a matter of concern to us."
Her comments came as Syria threatened to retaliate for an Israeli air attack and Iran said there will be repercussions for Israel over Wednesday's raid.
Clinton said that despite US efforts to bring Moscow on board to work for an international solution to the 22-month war in Syria that has claimed some 60,000 lives, Russia was continuing to prop up the regime.
Russians 'not passive'
"The Russians are not passive bystanders in their support for Assad. They have been much more active on a number of fronts," she said.
"Their defence of Assad in the Security Council has been the most public, visible sign of that. But there are other ways that they have tried to protect the regime," she added.
"We have reason to believe that the Russians continue to supply financial and military assistance in the form of equipment to Assad."
Al Jazeera looks at how support for the regime and the rebels has ebbed and flowed
The top US diplomat gave a grim assessment of the progression of the war, warning of "the dangers of an increasing civil war and a potential proxy war”.
"The worst kind of predictions of what could happen - both internally and spilling over the borders of Syria - are certainly within the realm of the possible now," she said.
"The killing goes on," she added, saying the US was "disturbed by increasing Iranian and Lebanese Hezbollah activities" in Syria.
But Clinton voiced hope Moscow might still change its stand, "because they cannot look at what is happening and not believe that it could be incredibly dangerous to everybody's interests, including theirs".
Joe Biden, US vice president, will be meeting with Sergei Lavrov, Russian foreign minister, on the sidelines of the Munich Security Conference in Germany opening on Friday.
Biden is also set to meet Syrian opposition leader Mouaz al-Khatib. It would be the highest-level US meeting with the Syrian opposition.