The United States has completed its military withdrawal from northeastern Syria, bringing the total number of US troops in the rest of Syria to about 600, Defense Secretary Mark Esper said.

He stressed that he retains the ability to bring in small numbers of troops and remove them as necessary in Syria. But he said the number of troops would fluctuate at 600 in the foreseeable future.

"The number will be relatively constant around that number, but if we see that things are happening ... I will be able to increase the number slightly," Esber said on Wednesday evening as he returned from the NATO summit on the outskirts of London. More if the Europeans contribute to the mission in Syria.

"The coalition is talking a lot again. Some allies may want to contribute troops," the minister said, without referring to any imminent new contribution.

"If an allied NATO member state decides to provide 50 personnel to us, I may be able to withdraw 50 people."

Esber's remarks may signal the end of a period of turmoil over the US military presence in Syria after President Donald Trump's initial withdrawal order in October.

Since Trump's announcement, US forces in Syria have fallen by about 40 percent. The number was about 1,000.