UN Secretary-General Antonio Guterres called on all parties in northeast Syria to exercise maximum restraint, his spokesman said on Monday after the United States began pulling troops back from the area, opening the way for a Turkish strike on Kurdish-led forces long allied to Washington.

"It's very important all parties exercise maximum restraint at this time," UN spokesman Stephane Dujarric told reporters.

"The Secretary-General also emphasizes that civilian and civilian infrastructure need to be protected at all times and that sustained, unimpeded and safe humanitarian access to civilians in need must be guaranteed," he said.