The United States has completed its military pullback in northeastern Syria, settling into a more stable posture of about 600 troops in the rest of the country after repositioning and reducing forces, Defense Secretary Mark Esper said.

Esper's remarks in an interview with Reuters could signal the end of a period of turbulence and uncertainty surrounding the U.S. military presence in Syria after President Donald Trump's initial withdrawal order in October.

Since then, troop levels in Syria have fallen about 40 percent from around 1,000.

Esper stressed he retained the ability to move in and out smaller numbers of forces as needed into Syria. But he suggested the number of troops will fluctuate around the 600-level for the foreseeable future.

"It will be relatively static around that number. But if we see things happen ... I can dial up a little bit," Esper said late on Wednesday during a flight back from the NATO summit on the outskirts of London.

Esper also didn't rule out being able to reduce U.S. troop levels in Syria further if European allies contributed to the Syria mission.

"The coalition is talking a lot again. We could see some allies want to volunteer troops," Esper said, without suggesting any new contribution was imminent.

"If an allied country, a NATO country, decided to give us 50 people, I might be able to turn off 50 people."

The U.S. military says it is focused on preventing a resurgence of Islamic State in Syria and carried out a raid last month that led to the death of Islamic State leader Abu Bakr al-Baghdadi.

Trump, in London, said he wanted remaining U.S. forces to ensure Syria's oil reserves don't fall back into the militant group's hands.

"We kept the oil. And the oil is what fueled ISIS (ISIL)," Trump said.

The same day, Pentagon also denied sending up 14,000 more troops to the Middle East in the face of a perceived threat from Iran amid news reports saying possible deployment may take place.

"To be clear, the reporting is wrong. The U.S. is not considering sending 14,000 additional troops to the Middle East," spokeswoman Alyssa Farah tweeted.

(With input from Reuters)