Syrian government troops have moved into towns and villages in northeastern Syria, including the flashpoint region of Manbij, setting up a potential clash with Turkish-led forces advancing in the area as long-standing alliances in the region began to shift or crumble following the pullback of US forces.
The Syrian military's deployment on Monday near the Turkish border came after 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, now in its sixth day.
Assad's return to the region his troops abandoned in 2012 at the height of the Syrian civil war is a turning point in Syria's eight-year civil war, giving yet another major boost to his government and its Russian backers.
It is like to endanger, if not altogether crush, the brief experiment in self-rule set up by Syria's Kurds since the conflict began.
The rapidly changing situation was set in motion last week, when US President Donald Trump ordered American troops in northern Syria to step aside, clearing the way for an attack by Turkey, which regards the Kurdish fighters as terrorists.
Since 2014, the Kurds have fought alongside the US in defeating the Islamic State in Syria, and Trump's move was decried at home and abroad as a betrayal of an ally.
Faced with unrelenting criticism, Trump said on Monday he was putting new sanctions on Turkey, halting trade negotiations and raising steel tariffs in an effort to pressure Ankara to stop its offensive.
Vice President Mike Pence also said Trump was sending him to the Middle East because the president was concerned about instability in the region.
Pence said Trump spoke with Turkish leader Recep Tayyip Erdogan earlier on Monday and called for an immediate end to Turkey's military campaign. He added that the US is "simply not going to tolerate Turkey's invasion of Syria any longer".
In the past five days, Turkish troops and their allies have pushed into northern towns and villages, clashing with the Kurdish fighters over a stretch of 200 kilometres. The offensive has displaced at least 130,000 people.
"Where is the United Nations? Let them come see the blood of our children on the floor! Why don't they show up?" cried a medic at the Tal Tamr hospital, which received dozens of injured people from nearby Turkish shelling in recent days.
Abandoned in the middle of the battlefield, the Kurds turned to Assad and Russia for protection and announced on Sunday night that Syrian government troops would be deployed in Kurdish-controlled towns and villages along the border to help repel the Turkish advance.
Kurdish official Aldar Khalil said in a statement that the aim of the agreement is for Syrian troops to be deployed along the border, except for the area between the towns of Ras al-Ayn and Tal Abyad, where Turkish troops are advancing.
The dramatic events are a crushing blow to the dreams of Syria's Kurds who had built up a degree of autonomy that was unthinkable before the war, when they were an oppressed minority by the Assad family rule.
Meanwhile, Turkey has warned its NATO allies in Europe and the United States not to stand in its way.
Erdogan has already said Turkey will not negotiate with the Syrian Kurdish fighters, saying they have links to a long-running Kurdish insurgency within its own borders.
The European Union unanimously condemned Turkey's military move and asked all 28 of its member states to stop selling arms to Ankara, Spanish Foreign Minister Josep Borrell told the AP.
Trump said the roughly 1,000 US troops he ordered out of Syria would remain in the Middle East to prevent a resurgence of the IS threat.
In Moscow, Kremlin spokesman Dmitry Peskov told reporters that Russian and Turkish officials have remained in close contact. Russia appeared to be working on de-confliction between Turkish and Syrian troops.
Australian Associated Press