A second round of UN-led peace talks, dubbed Geneva II, broke down in acrimony earlier this month, threatening international attempts to mediate an end to Syria's vicious civil war.
International envoys are trying to get the process back on track, but Washington accused Bashar al-Assad's regime of using strong-arm tactics to intimidate opposition negotiators.
"We call on the regime to immediately and unconditionally release all those unfairly arrested," State Department spokeswoman Jen Psaki said yesterday, expressing Washington's "outrage."
The impetus behind the Geneva dialogue came from a joint US-Russian diplomatic initiative, but the rival powers remain themselves deeply divided over the crisis in Syria.
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US Secretary of State John Kerry is due to meet his Russian counterpart Sergei Lavrov in Rome next week for their first face-to-face meeting since the last round of talks failed.
But he was critical of Moscow's stance, accusing the Kremlin of increasing its supply of weapons to Assad's forces.
"Frankly, Russia is increasing its assistance to Assad," Kerry said.
What Assad "is doing is outrageous, unconscionable, unacceptable, disgraceful, craven, it's horrendous. And we all know that. Everybody knows that," Kerry said, in an interview with MSNBC television.
One tangible result of the US-Russian diplomatic push was last year's agreement by Assad to hand over his chemical weapons arsenal for destruction by international monitors.
A fourth consigned of such arms - in this case mustard gas - was shipped put of the Syrian port of Latakia yesterday.
Rebel ranks have been bolstered by foreign Jihadi fighters, and the regime has the support of the Iranian-backed Lebanese Hezbollah militia, fuelling fears that the conflict may spread.
The presence of Al-Qaeda linked extremists among the opposition forces has raised concerns in the West, but Washington places the bulk of the blame for lack of progress in talks on Assad.