The United States is accusing Damascus of paralysing Geneva peace negotiations, while Russia denied that and said nations backing Syrian rebels were leaning toward trying to end the civil war on the battlefield rather than in talks.
A second round of talks in Geneva broke up on Saturday with chief mediator Lakhdar Brahimi lamenting a failure to advance much beyond agreement on an agenda for a third round later.
On Monday (local time) US Secretary of State John Kerry said President Bashar al-Assad's government was behind the impasse, aided and abetted by Russia and other allies of Damascus.
"The regime stonewalled. They did nothing except continue to drop barrel bombs on their own people and continue to destroy their own country. And I regret to say they are doing so with increased support from Iran, from Hezbollah and from Russia," he said in Jakarta during a trip to Asia and the Middle East.
Kerry appeared to be trying to tighten diplomatic pressure on Assad to reach a political settlement that would end government attacks on rebel-held areas and relieve the plight of tens of thousands of Syrians cut off from humanitarian aid.
Pressing Moscow to wring a more flexible stance from Assad, he said: "Russia needs to be a part of the solution", rather than helping the Syrian leader with arms and other support.
In Moscow, Russian Foreign Minister Sergei Lavrov hit back, citing "evidence that certain sponsors of the opposition are starting to create a new structure" bringing in Assad foes who have left the main opposition National Coalition.
"In other words, a course is being set to move away from the negotiations track and once again place bets on a military scenario," Lavrov said at a joint news conference after talks with his Eritrean counterpart.
He also faulted the United States for failing to ensure the presence of a broadly representative opposition delegation at the Geneva talks, saying that Russia had done its part in getting Assad's government to the table.
"Russia is always being urged to make more of an effort to resolve the Syrian conflict," Lavrov said. "When we hear that Russia must take some steps, it's necessary to remember one simple truth: We have done everything we promised."
"MAGNET FOR TERRORISTS"
US President Barack Obama said on Friday he was considering new ways to pressure Assad, but gave no details.
Russia has shielded Assad from Western and Arab pressure since the conflict began in March 2011, using its veto power to block UN Security Council resolutions and insisting that his exit from power cannot be a precondition for peace talks.
Moscow helped Syrian government negotiators resist discussion of a transitional governing body for Syria at the Geneva talks last week by suggesting it endorsed their demands that tackling "terrorism" top the agenda.
The Syrian government's efforts to make that a priority were "completely justified" because Syria "is increasingly becoming a magnet for jihadists and Islamic radicals of all stripes," the Russian Foreign Ministry said on Monday.
Russia has accused sponsors of the rebels of pushing for "regime change".
The conflict has drawn thousands of foreign fighters into Syria to fight either for the mostly Sunni Muslim rebels or for Assad, whose Alawite sect is an offshoot of Shi'ite Islam.
Fighting has killed more than 140,000 people - more than 7000 of them children - according to the Britain-based, pro-opposition Syrian Observatory for Human Rights, and is destabilising the country's neighbours.
Kerry derided the Syrian government's insistence that terrorism should be the main focus of the Geneva peace talks.
"Assad himself is a magnet for terrorists," he said, accusing him of pursuing "state-sanctioned terror against his own people" by indiscriminate bombing, starvation and torture.
The Russian foreign ministry praised what it called the "positive intent" of the government delegation at the talks and said Brahimi "should not stray toward unilateral accusations and place responsibility for the stalled dialogue on one side".
TRANSITION OR "TERRORISM"?
The Geneva talks were initiated by Lavrov and Kerry, who jointly announced in May that they would seek to bring Assad's government and its opponents together to seek a solution.
The sides are supposed to be seeking a solution based on a June 2012 document approved by Russia and the United States that called, among other things, for the creation of a transitional government body formed by mutual consent.
That wording all but rules out a place for Assad because the opposition has said he must not remain in power, but Damascus has indicated he may seek re-election later this year.
The Syrian opposition negotiators have little sway over rebels fighting on the ground, with hardline Islamist factions that share al Qaeda's ideology eclipsing more moderate groups.
More evidence of disarray in the Western- and Arab-backed Syrian Free Army (FSA) emerged at the weekend when it sacked its leader and replaced him with a more experienced field commander.
The FSA's Supreme Military Council said it had replaced General Selim Idriss with Colonel Abdelilah al-Bashir because of "the ineffectiveness of the command in the past few months".