"We think that Russia has a responsibility to press the Syrian regime to approach these talks with seriousness," the senior US official said on condition of anonymity.
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.
While a first round of talks in January was seen as a relative success for getting the parties to meet face-to-face for the first time in nearly three years of civil war, the foes this week have not even been able to agree on an agenda for the talks.
It did not react to a proposal put forward on Wednesday by the opposition delegation for a political transition in Syria, and has so far balked at UN mediator Lakhdar Brahimi's suggestion that the parties discuss the two issues in parallel.
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"The United States would like there to be more progress, for the regime to come to the negotiating table with more seriousness of purpose," the US official said.
While the opposition had "put several constructive proposals on the table," the official said the regime "has been stonewalling every step of the way."
"We hope and trust ... That Russia will urge the regime to engage in a serious and constructive way," the official added.
Asked whether there was an indication that Moscow was trying to get the regime delegation in Geneva to toe the line, the official pointed out that "the regime hasn't done so yet, and that speaks for itself."
It was unclear today if the second round of talks was officially over or might timidly continue into Saturday, or whether the parties would agree to come back for a third round of negotiations.
Brahimi himself vowed yesterday to keep trying, but added if there was no possibility to move forward he would "say so".