"The Iranians have made it clear for some time that keeping Assad in power was one of their highest priorities. We believe they have acted on that by sending in more personnel, not only to help Assad, but to support and advise military security forces," Clinton said.
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 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," she told a roundtable of journalists including AFP.
But Clinton also noted 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.
"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 defense 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."
And 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 United States 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."
Vice President Joe Biden will be meeting with Russian Foreign Minister Sergei Lavrov on the sidelines of the Munich Security Conference in Germany opening on Friday.
And Clinton offered assurances that her successor as secretary of state, John Kerry, would "pick up where I left off and do all that we can do... with respect to a political transition in Syria."
One of the issues was to help train the Syrian opposition as it tries to undertake and prepare for the day when Assad falls.
"You have to make it clear that there will be something other than hardened fighters when this conflict ends, otherwise it might never end in the foreseeable future because there will be so much to fight over," Clinton said.