Brussels, Belgium: Turkey is "heading in the wrong direction" with its incursion into Syria and its deal with Russia to jointly patrol a "safe zone" there, US Defense Secretary Mark Esper warned Thursday.

"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, Mark Esper told a conference in Brussels ahead of a NATO defence ministers' meeting.

"I think the incursion's unwarranted," Mark Esper said.

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 offensive 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.

Mark 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.

He added that "I was not about to put less than 50 US soldiers in between a 15,000-man-plus Turkish army preceded by Turkish militia and jeopoardise the lives of those servicemen". Nor was he "about to start a fight with a NATO ally," he said.

Mark Esper 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."

In his speech and question-and-answer session at the event hosted by the German Marshall Fund think tank, Mark Esper highlighted threats he said were posed by Iran, Russia and, especially and above all, China.

"NATO allies should be looking east," to China, he said, stating that Beijing's "heavy hand" was being seen politically, militarily and economically not only in Asia but further abroad, including into Europe with its "belt-and-road" project.

Adoption by NATO allies of China's Huawei company to build 5G telecoms networks would be a threat to intel-sharing within the North Atlantic Treaty Organisation, he said.

"I'll counsel our allies -- if Huawei becomes your provider of choice, this will affect our ability to share intelligence. We can't trust those networks," he said, adding: "We need to address those threats with eyes wide open."