Boris Johnson has has expressed his "grave concern" over Turkey's military action in northern Syria, Downing Street has said.
The Prime Minister spoke to President Recep Tayyip Erdogan by telephone and urged him to end the operation.
He urged the Turkish leader to enter into dialogue with a view to reaching a ceasefire.
A No 10 spokesman said: "He [Mr Johnson] expressed the UK's grave concern about Turkey's military operation in northern Syria, which he said could further worsen the humanitarian situation there and undermine the progress made against Daesh (so-called Islamic State).
"The Prime Minister underlined that Turkey is an important partner for the UK and a Nato ally. He recognised Turkey's role at the forefront of the fight against Daesh and its generosity in supporting refugees who've fled the civil war in Syria.
"But the Prime Minister was clear that the UK cannot support Turkey's military action. He urged the president to end the operation and enter into dialogue, and said the UK and international partners stand ready to support negotiations towards a ceasefire."
It comes after both France and Germany said they were halting arms exports to Turkey.
France is halting exports of any arms to Turkey that could be used in its offensive against Kurds in Syria and wants an immediate meeting of the US-led coalition against Islamic State extremists.
The nation's defence and foreign ministries made the announcement in a statement Saturday reiterating opposition to the Turkish military operation which is facing growing international condemnation.
While Germany's foreign minister has announced that the country will curtail its arms exports to Turkey.
Heiko Maas on Saturday told weekly Bild am Sonntag that, "against the background of the Turkish military offensive in northeastern Syria, the government will not issue any new permissions for any weapons that can be used by Turkey in Syria."
The main Kurdish-led group in northern Syria is calling on the United States to carry out its "moral responsibilities" and close northern Syrian airspace to Turkish warplanes.
Reydour Khalil, a spokesman for the Syrian Democratic Forces, read the group's statement, which did not directly name the U.S. but referred to them as "our allies."
Saturday's statement said "we don't want them to send their soldiers to the front lines and put their lives in danger."
