Turkey launched its ground and air offensive on Wednesday after US President Donald Trump cleared the way by pulling American forces back from the area.
Kurds fighting as the Syrian Democratic Forces (SDF) played a vital role alongside US troops in defeating Islamic State but now the US has been accused of abandoning them.
The Trump administration has since demanded that Turkey halts its offensive, which has seen American forces come under artillery fire, warning the regime that it was causing “great harm” to ties and could face sanctions.
On Saturday evening, Prime Minister Boris Johnson spoke with Turkey’s President Recep Tayyip Erdogan and joined calls for him to end the fighting.
A Downing Street spokesperson said: “[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 [IS].
“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 came after France announced it would stop selling weapons to Turkey that could be used in the conflict, also calling for an urgent meeting of the coalition against Islamic State extremists.
People in Paris protested against Turkey’s action against the Kurds
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The UN has estimated more than 100,000 people have been forced to flee the fighting and Turkey is reported to have taken Ras al Ain, a key border town, although this is disputed by the Syrian Kurdish forces.
Mr Erdogan says the military action is necessary for national security, arguing that the Kurds in northeast Syria are linked to a terrorist insurgency in Turkey.
Earlier on Saturday, he said: “We will never stop this step we have taken… We will not stop it no matter what anyone says.”
