US Secretary of State John Kerry slammed Russia on Monday for "enabling" Syria's President Bashar al-Assad to stay in power in the war-torn country, after Geneva peace talks broke off without result at the weekend.
The so-called Geneva II process was initiated by the United States, which backs the Syrian opposition coalition, and Moscow, which supports the government in Damascus.
But a second round of talks aimed at finding a political settlement to the three-year conflict ended in acrimony on Saturday, throwing the future of the negotiations into doubt.
On Monday, Kerry took aim at Russia, whom the US has previously accused of not doing enough to push its ally to engage in the talks.
The Syrian regime "stonewalled" at the Geneva talks and continued to "destroy their own country", Kerry told reporters during a visit to the Indonesian capital Jakarta.
"And I regret to say they're doing so with increased support from Iran, from Hezbollah and from Russia," he said.
"Russia needs to be a part of the solution and not be distributing so much more weapons and so much more aid that they're in fact enabling Assad to double-down. Which is creating an enormous problem."
Kerry noted that Russia "has stood up publicly with me on several occasions and said they're committed to that transition government... yet we have not seen the kind of effort to create the dynamic by which that can be achieved".
No date was set for third round of talks and it was unclear whether any would be held.
The UN's Syria envoy Lakhdar Brahimi noted at the weekend that the two sides had at least finally agreed on an agenda for possible future talks.
The opposition says the focus must be on creating a transitional government -- without Assad.
The regime representatives have insisted Assad's position is non-negotiable, and refused to discuss anything beyond the "terrorism" it blames on its opponents and their foreign backers.
At the weekend, Kerry blamed the Syrian regime's "obstruction" for the breakdown at the talks.
On Monday he also lashed out at Syria again, saying the regime "refused to open up one moment of discussion legitimately about a transitional government".
"Everybody should agree that the Assad regime's attitude is what has made these talks tougher," he added.
Brahimi said at the weekend if the sides returned, they would discuss violence and terrorism first, then the transitional governing body (TGB), followed by national institutions and finally national reconciliation and national debate.
However, he said the regime side baulked at his suggestion they spend one day on violence and the next on political transition, "which raises the suspicion of the opposition that the government doesn't want to discuss TGB at all".
The evacuation of civilians from besieged rebel-held areas of Homs -- seen as the talks' only tangible result so far -- has been hailed as a relative success.
The Red Cross, however, voiced alarm Saturday at the chaotic circumstances of the evacuation amid continued shelling and continuing attacks on aid convoys, while the UN has said more than 250,000 Syrians remain stuck in besieged communities.
Syria's uprising began as a series of peaceful pro-democracy protests in March 2011, but a brutal regime crackdown ignited a full-blown civil war.
More than 140,000 people have been killed in the conflict, according to the Syrian Observatory for Human Rights.