Turkey should show proportionality in its pursuit of those behind a failed military coup, German Chancellor Angela Merkel said on Thursday, adding that she was following developments in the NATO partner country with concern.

"Of course, when a putsch attempt like this takes place in a country it is important to take action against the rebels with all the means and potential of the constitutional state," Merkel told a news conference.

"In a constitutional state - and this is what worries me and what I am following closely - the principle of proportionality must be ensured by all," she added.