The Syrian Human Rights Association (ADHS) urged the government on Tuesday to stop making arbitrary arrests and pointed to a number of recent cases.
A statement from the group said it "condemned the continuation of these arbitrary arrests and all the flagrant human-rights violations".
ADHS said that a young Syrian man, Shadi Shiwaikh, was recently arrested on his return from the US along with four friends "for unknown reasons".
One month earlier, another national, Faris Hanna, was arrested by security services. The group said again it did not know why he was detained or where.
Meanwhile, lawyer and rights activist Anwar Bunni told journalists that writer and human-rights activist Salman Yusif had had his passport confiscated after returning from a trip to Europe.
According to estimates by various human rights organisations, Syria is holding around 2000 political prisoners, including 200 members of its Kurdish minority.