Under the headline "Set Her Free," Merkel and other well-known figures called for the safe return of 43-year-old Susanne Osthoff who with her driver has been missing since Nov. 25.
"The German government is doing everything it can to save the lives of Susanne Osthoff and her escort," Merkel wrote in the Bild am Sonntag newspaper. "We call on the perpetrators to release the hostages immediately."
Merkel also told public television late Sunday that the government had no new information on Osthoff.
"We do not yet know where she is and have not yet received signs of life from her," she said, adding that the government was using all the channels at its disposal "to save the life of Susanne Osthoff and I hope that that will be successful."
European Union foreign policy chief Javier Solana, leaders of the German Muslim and Jewish communities, opposition party chiefs and presidents of leading charity groups joined the appeal.
Nadeem Elyas, who heads Germany's Central Council of Muslims, also signed the appeal and offered himself as a hostage to the kidnappers in exchange for Osthoff.
"I'm considering everything that could save her live," he told die tageszeitung newspaper.
Still no contact with kidnappers
The kidnappers set a deadline last Tuesday of three days for Germany to stop training Iraqi police officers as the price for the hostages' release, weekly magazines Focus and Der Spiegel reported in advance copies of their Monday issues. The reports said the demand expired early Friday morning.
German Foreign Minister Frank-Walter Steinmeier said Saturday the government had failed to establish "direct or indirect" contact with the kidnappers but was working around the clock to secure their release.
Der Spiegel, quoting the government crisis team dealing with the case, said the kidnappers could be members of the Arab nationalist "Ishrin" groups.
It said Berlin was relying on Kurdish intermediaries and the Sunni leader Abd al-Muneim al-Badari to establish contact with the kidnappers.
Kurdish leader Massud Barasani was contacted as a go-between.