Around 150 Turkish officers have left NATO's command structure following the attempted military coup against President Recep Tayyip Erdogan, the alliance's head of forces in Europe said on Wednesday.

"The Turkish military had a fairly sizeable number of officers that were either detained, some recalled from here, that worked for us here, and some retired from active service," US General Curtis Scaparrotti told journalists on the sidelines of a NATO meeting in Brussels.

In all, there was "about a 50 percent reduction" in the number of Turkish officers, or about 150 individuals, he said.

"I have refilled a little over half of that and there are more officers coming in," he said.

"It does have an impact because it was largely very senior personnel and you lose a good deal of experience.

"So we are seeing a bit of degradation there and we are also having to build relationships with new leaders, NATO partners. That's coming along fine but it is going to take some time... it puts an extra load on our remaining people."

Turkey has carried down a wide-ranging crackdown on its military following the botched coup of July 15.

It has arrested or discharged thousands of army personnel and detained more than a hundred generals and admirals.

On November 18, NATO chief Jens Stoltenberg said a number of Turkish officers serving in NATO command positions had requested asylum.

Erdogan, two days later, warned NATO against sheltering "terrorist" soldiers "involved in plotting a coup".

In a high-profile case, eight Turkish officers have sought asylum in Greece, a move that has strained relations between the two neighbours.

On Tuesday, a Greek court ruled that three should be sent back. On Monday, a request to extradite three others was turned down on the grounds that the Turkish authorities had not provided sufficient evidence, and that their personal safety was in jeopardy at home.

Scaparrotti admitted to being "concerned" about the safety of Turkish officers sent back to their homeland.

"Will they (the Turkish regime) follow the rule of law and treat their people appropriately?

"In some cases I really don't understand what their (the officers) future is at this point."