"The president's been very clear about what he wants to see out of President Erdogan," he added.

US President Donald Trump on Thursday hailed "great news" following talks between his vice president and Turkish leader Recep Tayyip Erdogan on a ceasefire in northern Syria.

"Great news out of Turkey," Trump tweeted, shortly before Vice President Mike Pence announced in Ankara that Turkey had agreed to end its military operation once Kurdish fighters withdraw from a safe zone.

"Millions of lives will be saved!" Trump wrote.

Trump's chief of staff had earlier said the president may revoke a White House invitation extended to Erdogan, depending on the outcome of the talks in Ankara.

"It's still on the schedule," Mike Mulvaney told reporters, "but I think that's one of those wait and see things."

"The president's been very clear about what he wants to see out of President Erdogan," he added.