Turkish President Recep Tayyip Erdogan has rejected a US call for an immediate ceasefire in northern Syria, saying Turkey’s offensive will continue.
His comments come as US Vice-President Mike Pence and US Secretary of State Mike Pompeo prepare to fly to Turkey to try to broker a truce.
On Tuesday, Russia warned that it would not allow clashes to take place between Turkish and Syrian forces.
Turkey says its offensive aims to push Kurdish forces from the border region.
It considers elements of the Kurdish-led Syrian Democratic Forces (SDF) a terrorist organisation.
Ankara also says it wants to create a “safe zone” reaching about 30km (20 miles) into Syria to resettle up to two million Syrian refugees currently in Turkey.
However, Syrian forces – which are backed by Russia – advanced north at the weekend following a deal with the Kurds to try to hold back the Turkish operation.
The withdrawal of US troops from the region, announced last week, gave Turkey a “green light” for the offensive, critics of the Trump administration say.
The US has repeatedly denied this, and on Monday Washington announced sanctions on Turkish ministries and senior government officials.
Dozens of civilians have been killed in the operation so far and at least 160,000 have fled the area, according to the UN.