The tanks are thought to have been used by Turkey against the Islamic State (IS) group in Syria. However, defence experts have claimed in German media that recent images from Turkey's "Operation Olive Branch" appear to show them being used against Kurdish groups.
Some German politicians have requested that any moves to approve a tank upgrade deal be halted.
The row comes just weeks after the two countries' foreign ministers vowed to improve bilateral ties. Relations between the two Nato members have soured dramatically in recent years.