"I've done what was possible to do," Clinton told reporters on the eve of her last day as secretary of state.
But she painted a harrowing picture of a war that could still get worse.
"The worst kind of predictions about what could happen internally and spilling over the borders of Syria are certainly within the realm of the possible now," she said.
The conflict "is distressing on all fronts," Clinton told a roundtable of journalists Thursday, a day before John Kerry is sworn in as her successor. She pointed the finger primarily at Iran, accusing it of dispatching more personnel and better military materiel to President Bashar Assad's regime to help him defeat rebel forces. Its Lebanese proxy, Hezbollah, is also playing a bigger role in the conflict.
"The Iranians are all in for Assad, and there is very little room for any kind of dialogue with them," Clinton said.
She spoke after Syria threatened Thursday to retaliate for an Israeli airstrike, and its ally Iran warned ominously that the Jewish state would regret the attack.
In a letter to the U.N. secretary-general, Assad's regime stressed its "right to defend itself, its territory and sovereignty" and holding Israel and its supporters accountable. And Ali Abdul-Karim Ali, Assad's ambassador in Lebanon, said his government maintained "the option and the capacity to surprise in retaliation."
Clinton declined to talk specifically about Israel's strike, which U.S. officials described as targeting trucks containing sophisticated Russian-made SA-17 anti-aircraft missiles. The trucks were next to a military research facility, and the strike hit both the trucks and the facility, U.S. officials said.
If the SA-17s were to have reached Hezbollah, they would have greatly inhibited the Israeli air force's ability to operate in Lebanon, where Israel has flown frequent sorties in recent years. The attack has inflamed regional tensions already running high over Syria's 22-month-old civil war, and which has already led to deaths in neighboring Turkey and Lebanon.
In her strikingly candid assessment, Clinton spread the criticism to Russia, which has stymied U.S.-led efforts to set global sanctions against the Syrian regime at the U.N. Security Council. Washington and Moscow have remained in a three-way dialogue with the U.N. special envoy for Syria, Lakhdar Brahimi, since late last year, but Clinton said the Russians were simultaneously providing financial assistance and military equipment to Assad.
"The Russians are not passive bystanders in their support for Assad. They have been much more active," she told reporters. "But maybe they will change. And maybe they will be more open to an international solution because they can't look at what's happening and not believe it could be incredibly dangerous to everyone's interests, including theirs."
Despite the dismal outlook of the war, Clinton stressed she in no way has softened her opposition to the United States providing weapons to Syrian rebels or intervening militarily to halt the conflict. Asked about America's Gulf allies who have sent arms to the Syrian opposition, Clinton said the Obama administration continues to urge caution on the types of materiel being supplied and vetting recipients.
The U.S. fears that if extremist groups get dangerous weapons, they could then use them against American interests or Israel.
"Sitting here today, I can't tell you that we've been entirely successful in that," Clinton said. "There are those who are supplying weapons and money for weapons, who really don't care who gets it as long as they are against Assad - and who have the view that once Assad is gone, then we'll deal with the consequences of these other groups who are now armed and funded. That's not our view."
She stressed that a political solution was necessary, and defended Syria's top opposition leader for suggesting earlier this week that he'd be willing to negotiate with members of Assad's regime. The call provoked an outcry from rebels who insist that Assad must step down first.
And she urged Kerry to press on with efforts at the United Nations and elsewhere to "create more credibility for the opposition" and create the possibility for more forceful international action to end the war.
"I think I've done what was possible to do over the last two years in trying to create or help stand up an opposition that was credible and could be an interlocutor in any kind of political negotiation," Clinton said. "We've engaged in a steady drumbeat of activities and trying to put together a coalition and trying to find a way to get something through the Security Council that would serve as the international legal basis for further action to be taken by many countries."