Video report by ITV News Correspondent Emma Murphy
Boris Johnson has expressed "grave concern" about the situation in northern Syria, calling on Turkey to end its military operation in the area.
In a telephone call to the Turkish leader, the Prime Minister voiced his "grave concern" that the action could worsen the humanitarian situation in the region and undermine the fight against so-called Islamic State (IS).
Mr Johnson called on him to enter into dialogue with a view to reaching agreement on a ceasefire.
A Number 10 spokesman said: "He 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 spokesman added: "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."
The incursion against the Syrian Kurds – who played a key role in dismantling the IS caliphate but are regarded as terrorists by Turkey – has been widely condemned by Western powers.
There are concerns that hundreds of suspected IS prisoners, including foreign fighters being guarded by Kurdish forces, may escape during the chaos.
Earlier Foreign Secretary Dominic Raab urged Ankara to show “maximum restraint” as the air and ground offensive enters its fourth day, with tens of thousands fleeing their homes in north-east Syria, according to the United Nations.
Speaking at the Nato Parliamentary Assembly at the QEII centre in Westminster on Saturday, Mr Raab said: “Our position is crystal clear. This incursion is wrong. We want to see maximum restraint, and avoid taking our eye off the ball with Daesh (IS).
“My fear is that the risk is that the humanitarian situation … could be made worse.”
The continued push by Turkey into Syria comes days after President Donald Trump cleared the way for Turkey’s air and ground offensive, pulling back US forces and saying he wanted to stop getting involved with “endless wars”.
His decision has drawn heavy criticism in the US amid accusations he was endangering regional stability and risking the lives of Syrian Kurdish allies.