"It has been 45 days since three members and employees of Grünhelme were kidnapped overnight on May 14-15, in the village of Harem in northwest Idlib province near the Turkish border," the group's founder Rupert Neudeck said.
The three staff are Bernd Blechschmidt, an engineer named only as Simon S., a builder, and 72-year-old Ziad Nouritwo, who had arrived in Syria just three days before he was kidnapped.
The aid group said it had contacted the German foreign ministry, police and Syrian exiles in Germany in a bid to obtain news of its employees, but to no avail.
The foreign ministry confirmed that three Germans had been "reported missing" in Syria and told DPA on Saturday that a crisis team was "working intensively on a solution."
Neudeck said his group was "very surprised to see that... the media in Germany have been silent" on the case.
"But we are no longer able to remain silent," he said.
The aid group urged German authorities, the United Nations and the European Union to "do everything" to find its employees and to secure their liberation.
Grünhelme (Green Helmets) has been in northern Syria for several months to help the reconstruct local infrastructure and to provide healthcare.
More than 100,000 people have died over the past 27 months in the Syrian conflict, which morphed from a popular movement for change into an insurgency after the regime unleashed a brutal crackdown on dissent.