The German government is facing calls to halt arms exports to Turkey after reports emerged that German-made Leopard tanks were being used in an offensive against the Kurdish YPG.
Some German politicians have requested that any moves to approve a tank upgrade deal be halted.
Turkish-led forces began an assault in Syria's north-west on Saturday.
The row comes just weeks after the two countries' foreign ministers vowed to improve bilateral relations.
Relations between the two Nato members have soured dramatically in recent years.
Reports on Friday suggested Berlin was moving to approve a request from Turkey for German arms manufacturer Rheinmetall to retrofit Leopard 2 models to better protect them from explosives.
They are thought to have been used by Turkey against the Islamic State (IS) group in Syria, but defence experts have speculated in German media that recent images from Turkey's "Operation Olive Branch" appeared to show them being used against Kurdish groups.
Politicians from both the left and right have spoken out against the tank upgrade deal, and have asked the government to clarify its position on the Turkish military offensive in the Afrin region of northern Syria.
Norbert Roettgen, a member of Chancellor Angela Merkel's Christian Democratic Union (CDU) party and chairman of the parliamentary foreign affairs committee, said it was "completely obvious" that Germany should not provide the upgrades at this time.
Agnieszka Brugger, a Green party MP, said the situation should be a "wake-up call" for the German government.
"An immediate halt to all arms exports to Turkey is long overdue," she told the Heilbronner Stimme newspaper.
German Foreign Minister Sigmar Gabriel said on Monday that he had called his Turkish counterpart to express concerns about the humanitarian impact of the Afrin offensive.
Turkey's president has vowed to "crush" the People's Protection Units (YPG) militia, which controls Afrin and more than 400km (250 miles) of Syria's northern border.