"Turkey put us all in a very terrible situation" by sweeping into northern Syria this month to fight Kurdish militia allied with the US in the fight against the Islamic State group, Esper told a conference in Brussels ahead of a NATO defence ministers' meeting.

The onus was on Turkey's NATO allies to now "work together to strengthen our partnership with them, and get them on the trend back to being the strong reliable ally of the past," he said.

The issue of Turkey's military operation in Syria is set to dominate the two-day NATO meeting, with diplomats in the organisation saying "frank" discussions with Ankara's representatives have already taken place.

A subsequent arrangement with Russia to clear Kurdish militia that Turkey regards as "terrorists" linked to the outlawed PKK group on its soil has also raised hackles.

Yet, while isolated in NATO, Turkey's strategic position between Europe and the Middle East is seen as too important to jeopardise, so the other alliance members have limited themselves to criticism only.

Esper defended the US decision to pull US forces out of northern Syria, effectively opening the path to the Turkish operation.

"The US decision to withdraw less than 50 soldiers from the zone of attack was made after it was made very clear to us that President (Recep Tayyip) Erdogan made the decision to come across the border," he said, adding that he would not "jeopardise the lives of those servicemen" nor "start a fight with a NATO ally".

He acknowledged "there has been some criticism" about the US withdrawal "but nobody's yet offered a better alternative to what the United States did. We are trying to keep a very strategic perspective."