Smoke rises from Syrian city of Ras al Ain following an attack from Turkey (Picture: PA).
The Kurds of northern Syria say they have done a deal with President Assad's government in an attempt to stop the Turkish invasion of their territory.
Syria said its army would deploy along the border to help the Kurdish-led Syrian Democratic Forces (SDF), hours after US President Donald Trump ordered the withdrawal of all American forces still in the region.
The US committed to removing troops from north-east Syria last week, leaving behind the Kurdish forces who fought alongside them against the Islamic State group.
The Kurdish announcement of Syrian support represents a major shift in alliances for the group - who form the bulk of the (SDF) and are considered terrorists by Turkey.
Syria's state news agency, Sana, says government forces have entered the town of Tal Tamr, approximately 12 miles from the border, to "confront the Turkish aggression".
Mr Trump said on Twitter that the US has been "very smart not to be involved in the intense fighting along the Turkish Border, for a change".
He added the US is using its economic power for world peace.
Speaking to American broadcaster CBS, US Defense Secretary Mark Esper said most of the 1,000 US troops were withdrawing from northern Syria due to the dangers posed in the area.
"We have American forces likely caught between two opposing advancing armies, and it's a very untenable situation", he said.
Last Friday, a small number of US troops were unhurt after Turkish artillery fire at an observation post in the north.