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Syria's warring sides traded barbs over the failure to achieve concrete results at peace talks in Geneva, amid doubts over the regime's participation in a planned new round in February.
No ceasefire was agreed, talks on a transitional government never began, and a deal to allow aid into besieged rebel-held areas of the central city of Homs went nowhere.
Clouding the horizon further, the Syrian Observatory for Human Rights said nearly 1,900 people had been killed since the talks began a week ago.
Washington also warned Syrian President Bashar al-Assad he could face consequences for failing to live up to international agreements on removing his chemical weapons stockpile.
Back in Geneva, the week of closed-door negotiations ended with the opposing sides continuing to spar over who is to blame for the bloody conflict that has claimed 130,000 lives.
UN mediator Lakhdar Brahimi, who orchestrated the first meeting between the two sides since the conflict erupted in March 2011, said he aimed to host a second round of talks starting on February 10.
But Syrian Foreign Minister Walid Muallem said there were no "tangible results" from the Geneva talks, and that Assad and his government would ponder whether it made sense to return for a second round.
That sparked criticism from key opposition supporter the United States.
"The regime continues to play games," State Department spokesman Edgar Vasquez said.
Opposition chief Ahmad Jarba confirmed his team would be back, even though sitting down with the regime for the first time since the start of the war was like "drinking from a poisoned chalice".
But he stressed that its presence was conditional on receiving "the means to defend our people on the ground."
"The pace of supporting our revolutionaries is quickening, as you may have heard in recent days," he said.
Unconfirmed media reports this week alleged that the US Congress had secretly approved resuming weapons deliveries to "moderate" Syrian rebel factions.
Working to rally support, Jarba was scheduled Tuesday to visit Russia, Assad's main ally on the global stage.
A senior US official hailed the planned visit, stressing that it indicated "the Russian side recognises that the opposition National Coalition also has a role to play in resolving the Syrian conflict."
Brahimi has urged the United States and Russia, which struggled for eight months to get the warring parties to the table, as well as others to exercise their influence to end the bloodshed in Syria.
