Turkey has arrested a Turkish lawyer hired by the German Embassy in Ankara, a diplomat at the Foreign Ministry said on Wednesday, a measure that could further raise tensions between the NATO partners.

Relations between the two countries have been strained over German criticism of President Tayyip Erdogan’s crackdown on suspected opponents after a failed coup in 2016 and Turkey’s military offensive last month against Kurdish militia in Syria.

“We are intensively trying to find out what the accusations are and to lift the remand,” said the German official, declining to be named. “The detention is incomprehensible.”

Read more: Reuters