The Syrian government has reportedly arrested relatives of opposition delegation members to peace talks in Geneva on accusations of "terrorism," the US said late Wednesday.
The US State Department expressed its "outrage" over the Assad regime's move and called for the release of the detained family members of opposition envoys.
"We call on the regime to immediately and unconditionally release all those unfairly arrested," State Department spokeswoman Jen Psaki said.
The US condemned the Damascus government's "new behavior" of attempting to "silence dissent".
"By targeting family members of opposition delegates participating in the Geneva talks, the regime is not only defying the international community but also seeking to suppress the legitimate aspirations of the Syrian people," she added.
"Political and arbitrary imprisonment, and the systematic torture and death of tens of thousands of people without access to due process, are but a few of the regime’s documented human rights violations," Psaki said.
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.
US Secretary of State John Kerry said President Bashar Assad's government was behind the impasse, aided and abetted by Russia and other allies of Damascus.
Reuters contributed to this report.