BERLIN (Reuters) - Germany has ruled out giving Turkey unfiltered access to imagery gathered by Tornado fighter jets operating out of Incirlik air base in southern Turkey as part of the broader fight against Islamic State militants, the Defence Ministry said.

German Tornado jets are pictured on the ground at the air base in Incirlik, Turkey, January 21, 2016. REUTERS/Tobias Schwarz

German lawmakers, concerned that Turkey could use the high-resolution aerial imagery in its military campaign against autonomy-seeking Kurds, have put strict limits on how German forces can share the data they gather.

A Defence Ministry spokesman declined comment on a Der Spiegel magazine report quoting a German diplomatic cable as saying that Turkey had linked its approval of German housing investments at the air base to getting access to the imagery.

The issue, emerging a few days before Chancellor Angela Merkel is due to travel to Turkey to meet President Recep Tayyip Erdogan, could cause further strains in the already frayed relationship between the two NATO allies.

The cable described Turkey’s push to tie the imagery issue to German plans to build new housing at the base as “blackmail”, according to the magazine report. It said two senior Turkish officials, a senior military officer and Erdogan’s policy adviser Bo Arslan had personally issued the demand.

The ministry spokesman said the German air force flew its surveillance missions in strict conformity with the underlying parliamentary mandate and provided the imagery “solely to the anti-Islamic State coalition”.

He said the German military followed a careful process to ensure the data was not misused for other purposes.

A German officer based in Qatar accepted only those missions aimed at the anti-Islamic State fight, while another German officer based at Incirlik reviewed the images before releasing them to the coalition, the spokesman said.

German lawmakers last year voted to extend support for the U.S.-led fight against Islamic State through the end of 2017, including deployment of over 250 soldiers at Incirlik. But this was only after Ankara reversed its position and allowed German lawmakers to visit the base.

Turkey had angered Berlin by temporarily blocking the visits after German lawmakers voted last June to declare the 1915 massacre of Armenians by Ottoman forces a “genocide.”

The two countries have also been at odds over Turkey’s crackdown on dissidents after the failed July 15 coup, and Turkey’s claims, which Berlin rejects, that Germany is harbouring Kurdish militants and other enemies of the Turkish state.

In addition, Turkey’s defence minister on Sunday was quoted as saying that Germany should reject requests for asylum from some 40 former Turkish soldiers that Ankara suspects of having links to the abortive coup.