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.
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.
A crisis also for Nato
The crisis in north-eastern Syria is also a crisis for Nato with both practical and political implications. The immediate fear is that much of the progress made towards defeating IS could be undone.
President Trump's willingness to throw the Kurds to the wolves has not gone down well with many allies; France, Germany and perhaps less stridently, Britain, have all urged the Turks to halt their operation. Spain has threatened to pull out its Patriot missile battery in Turkey in protest.
Mr Trump, who did little to try to stop the operation, has equally threatened to "totally destroy and obliterate" the Turkish economy if they go too far – an extraordinary statement coming from one Nato member to another.
Nato Secretary-General Jens Stoltenberg says that despite his deep concerns, Turkey will remain an important member of the alliance. He insists that Nato will get over its current divisions. But if you add to this crisis Ankara's recent decision to purchase Russian surface-to-air missiles, there is a clear sense that Turkey is slowly drifting away from it.
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.
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