UK Prime Minister Boris Johnson spoke with Turkish President Recep Tayyip Erdogan on Saturday evening, expressing grave concern about the ongoing military operation in northern Syria.

Johnson said that the United Kingdom does not support Ankara's 'Operation Peace Spring' and urged Erdogan to stop the invasion, calling for the start of a dialogue, Reuters reported.

"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 [...] 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", a spokesman said after a telephone call between the two leaders on Saturday, cited by Reuters.

Erdogan announced earlier this week the launch of his 'Operation Peace Spring' in the northern part Syria, reportedly against the Daesh terrorist organization and the Kurdistan Workers' Party (YPG/PKK), the latter designated by Ankara alongside Daesh as terrorists. From the Syrian side of the border, the territory is controlled by Kurdish-led Syrian Democratic Forces (SDF), classified by Ankara as a terrorist organization.

The Turkish military invasion of Syria is part of Ankara's long-standing goal to clear its border area of both Kurdish militias and Daesh terrorists to create a so-called safe zone where Turkey seeks to relocate a portion of the estimated 4 million Syrian refugees currently in its territory.

The Syrian government does not recognize the legitimacy of the Kurdish autonomy in the country's north-east, although Damascus has repeatedly condemned Turkey's occupational policies in the area.

Operation Peace Spring has been condemned by most Arab nations and the global community, with Washington subsequently threatening to impose tough economic sanctions against Ankara.

Turkish Foreign Minister Mevlut Cavusoglu said earlier that Operation Peace Spring was launched "in accordance with international law, Article 51 of the UN Charter and UN Security Council Resolutions on fight against terrorism".