The United States on Wednesday accused the Syrian government of undermining the Geneva peace talks by rounding up and arresting the relatives of opposition delegates, Agence France-Presse reported.
"We call on the regime to immediately and unconditionally release all those unfairly arrested," AFP quoted State Department spokeswoman Jen Psaki as saying, expressing Washington's "outrage."
She added: "The opposition delegation must be permitted to safely and securely work towards the political transition."
The U.S. accusing Syria comes after a second round of U.N.-led peace talks, dubbed Geneva II, broke down in acrimony earlier this month.
The collapse of the talks threatened international attempts to mediate an end to Syria's vicious civil war.
This has pushed international envoys to try to get the process back on track, but Washington accused Bashar al-Assad's regime of using strong-arm tactics to intimidate opposition negotiators.
The impetus behind the Geneva dialogue came from a joint U.S.-Russian diplomatic initiative, but the rival powers remain themselves deeply divided over the crisis in Syria.
U.S. 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.
"I do not find that constructive in the effort to try to get him to change his mind and be able to come to a decision that he needs to negotiate in good faith."
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
One tangible result of the U.S.-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 out of the Syrian port of Latakia on Wednesday.
But the disarmament process is behind schedule and meanwhile fighting continues in what is now a many-sided battle between opposition rebels, Islamist guerrillas and loyalist forces.
On Wednesday, Syrian state media said that the Assad troops killed at least 175 rebel fighters in an ambush on the outskirts of Damascus.
According to activists, Lebanon's Shiite militant group Hezbollah led the assault, joining hands with Syria's government forces.