US President Donald Trump has told the Turkish leader Recep Tayyip Erdogan to end the incursion into Syria, as the US announced sanctions against Turkey.

Vice-President Mike Pence said Mr Trump spoke directly to President Erdogan, demanding an immediate ceasefire and talks with Kurdish forces.

"President Trump communicated to him very clearly that the United States of American wants Turkey to stop the invasion, implement an immediate ceasefire and to begin to negotiate with Kurdish forces in Syria to bring an end to the violence," Mr Pence said.

He, along with the US national security adviser Robert O'Brien, is being sent to Ankara as soon as possible to try to negotiate an end to the fighting.

Turkey's incursion into northern Syria has raised international alarm - and came after Mr Trump's surprise move last week to pull a group of US forces, who had fought alongside Kurdish militia against IS, from a section of the border.

The US withdrawal freed Turkey to begin operations against the Kurds in Syria - action which it considers a matter of survival, and insists it will not tolerate the virtual self-rule that the Kurds succeeded in carving out in northern Syria.

Ankara wants to create a corridor - a so-called "safe zone" - clearing out the Kurdish forces.

The latest conflict has sparked yet another humanitarian crisis in the region with tens of thousands of people fleeing the fighting.

A crowd applaud as US President Donald Trump concludes his speech at the Values Voter Summit in Washington | Image: Manuel Balce Ceneta/AP/Press Association Images

UNICEF estimates that nearly 70,000 children have been displaced since hostilities escalated nearly a week ago.

It has confirmed that at least four children have been killed and nine others injured in northeast Syria.

Seven children have reportedly also been killed in Turkey.

The humanitarian group is concerned that at least 170,000 children could need humanitarian assistance as a result of ongoing violence in the area.

"As violence continues to escalate, UNICEF renews its calls on all parties to the conflict and those who have influence over them to protect children at all times", it says.

"Those fighting in the northeast and elsewhere in Syria must protect civilian infrastructure and not use it for military gains."

The US has strenuously denied that the US pull out amounted to a green light for the Turkish incursion.

A senior US official said on Monday: "This was not caused by any action of President Trump... Nothing we did was going to deter the Turks from what they wanted to do. President Erdogan was going to act regardless."

Faced with the Turkish onslaught, Syrian Kurdish forces previously allied with the US said they had reached a deal with President Bashar Assad's government to help them fend off Turkey's invasion.

Syrian government troops have already moved into towns and villages in northeastern Syria.