Turkey has refused German lawmakers permission to visit soldiers serving at an air base near the city of Konya, marking a new escalation in tensions between the two NATO allies.

Turkey's earlier refusal to let lawmakers visit German soldiers serving at Incirlik air base led to Berlin relocating those troops to Jordan. In Konya, German airmen contribute to a NATO air surveillance mission.

"We regret Turkey's request to delay the journey of a parliamentary delegation," a German foreign ministry official said on Friday. "We are in intensive talks with all parties, including NATO, to set a new date as soon as possible."

Germany's armed forces are under parliamentary control, for reasons rooted in its modern history, and Berlin says the lawmakers must have access to its soldiers. Germany relocated its troops from Incirlik after repeated attempts to gain lawmaker access.

In May Turkey authorized a visit to Konya, but then revoked its permission due to increased bilateral tensions, said Rainer Arnold, defense spokesperson for the Social Democrats (SPD), the junior party in Chancellor Angela Merkel's governing coalition.

The two countries are at odds over a host of issues, including Berlin's refusal to extradite asylum seekers Ankara accuses of involvement in last year's failed coup against President Recep Tayyip Erdogan, while Berlin is demanding the release of an imprisoned Turkish-German journalist.

Merkel's criticisms of Ankara have been muted, partly because she needs its cooperation in upholding a deal whereby Turkey prevents migrants and refugees, many fleeing Syria's civil war, from leaving its shores for the European Union.

With a German parliamentary election due in September, relations with Turkey are an especially sensitive issue, exposing her to criticism from other parties that hope to unseat her.

"If parliament can't visit, the German army cannot remain in Konya," said Thomas Oppermann, a senior SPD lawmaker in a statement. "The government must quickly find a solution."