According to German daily Neuen Osnabrücker Zeitung on Tuesday, the health of the 42-year-old hostage is declining.
The paper says it has been in contact with Afghan government insiders, who have warned the German government against “a renewed solo attempt” to pay ransom demands, which would embolden the kidnappers to commit further criminal acts.
A spokesperson from the German Foreign Ministry would not comment on the situation, the paper said.
Harald Kleber - who is married to an Afghan woman and carries an Afghan passport - was kidnapped in mid-December in the western province of Herat. Abductors recently demanded $50,000 ransom for his release.
Kleber has been kept "somewhere" between Herat and Farah province for the past three months.
The German national worked for a humanitarian organization in Herat between 2003 and 2004 and then stayed in the country.
A German prosecutor said in December that an arrest warrant had been issued for him over allegations of computer fraud in his native country.