Syria’s army has started to reach the north of the country, hours after the government agreed to help Kurdish forces facing Turkey.
State media said government forces, which are backed by Russia, had entered the strategic town of Tal Tamer, 30km (19 miles) south of the Turkish border.
The deal came after the US, the Kurds’ main ally, said it would withdraw its remaining troops from northern Syria.
Turkey’s offensive aims to push Kurdish forces from the border region.
Areas under the control of the Kurdish-led Syrian Democratic Forces (SDF) came under heavy bombardment over the weekend, with Turkey making gains in the key border towns of Ras al-Ain and Tal Abyad.
Dozens of civilians and fighters have been killed on both sides.
On Sunday, US Defence Secretary Mark Esper announced the Pentagon was moving up to 1,000 troops away from the north, citing fears that US forces would end up stuck between “two opposing advancing armies”.
The Turkish offensive and US withdrawal have been internationally criticised, as the SDF were the main allies of the West in defeating the Islamic State (IS) group in Syria. There are fears about a possible resurgence of the group and the escape of prisoners amid the instability.
What is known about the deal?
According to the Kurdish-led administration in northern Syria, Sunday’s agreement will allow the Syrian army to deploy along border areas controlled by Kurdish forces to “repel [Turkish] aggression”.
It is the first time government troops will enter those areas since 2012, when forces loyal to President Bashar al-Assad withdrew to fight rebels elsewhere, letting Kurdish militias take control.
Despite disagreeing with Kurdish attempts to self-rule, Mr Assad did not seek to retake the territory, especially after the Kurds became crucial partners in the international coalition against IS, with US troops in the region.
The deal represents a significant shift in alliances for the Kurds, who said they had been “stabbed in the back” by President Donald Trump after he pulled dozens of US troops from pockets in the north-east last week.
The move effectively paved the way for the operation by Turkey, which views elements of the Kurdish groups in Syria as an extension of the banned Kurdistan Workers’ Party, which has fought for Kurdish autonomy in Turkey for three decades.
Apart from being important partners in fighting IS in Syria, the Kurds were fundamental for Washington in limiting the influence of Russia and Iran, key allies of President Assad and US rivals, and keeping some leverage on the ground.
For now, Syrian forces will not be deployed between Tal Abyad and Ras al-Ain, where Turkey has focused its efforts. Apart from Tal Tamer, government troops also entered Ain Issa, according to state media, which broadcast pictures of residents celebrating their arrival.
What has the international reaction been?
On Monday, Turkish President Recep Tayyip Erdogan said Turkey would not back down its offensive “no matter what anyone says”, adding that the operation would continue until “ultimate victory is achieved”.
Turkey wants to push Kurdish forces away from a “safe zone” reaching 32km into Syria, to where it will resettle up to two million Syrian refugees currently in Turkey. Many of them are not Kurds and critics warn this could lead to ethnic cleansing of the local Kurdish population.
The Russian government, a close ally of Mr Erdogan, said it did not want to entertain the possibility of a clash between Russian and Turkish forces in Syria, and said it was in regular contact with Turkey’s authorities.
Earlier, President Trump suggested Kurdish forces might have released IS prisoners on purpose to draw the US into the conflict. “Big sanctions on Turkey coming!” he added, without giving details.
In other developments:
- The EU’s Foreign Affairs Council called on Turkey to withdraw its forces from northern Syria, saying the offensive “seriously undermines the stability and the security of the whole region”
- EU countries committed to suspending arms exports to Turkey but stopped short of an EU-wide arms embargo; Ankara said it would examine its co-operation with the EU due to its “unlawful and biased” attitude
- UN Secretary-General Antonio Guterres called for an immediate de-escalation of hostilities
What is the situation on the ground?
At least 50 civilians have been killed inside Syria and 18 over the border in southern Turkey, reports say. Kurdish forces have confirmed the deaths of 56 of their fighters while Turkey says four of its soldiers and 16 pro-Turkish Syrian fighters have been killed in Syria.
Up to 160,000 civilians had been displaced, according to UN humanitarian agency OCHA, which said the number was expected to rise.
The fighting has also spilled over to areas close to IS detainee camps, and officials at the Ain Issa camp said nearly 800 relatives of foreign IS members had escaped. The camp holds about 12,000 displaced people, previously including nearly 1,000 foreign women and children with jihadist links.
Turkey has said it will take responsibility for IS prisoners it finds during its offensive.
On Sunday, President Erdogan said his forces had already captured 109 sq km (42 square miles) of territory, including 21 villages.