Syrian forces took rapid advantage of an abrupt U.S. retreat from Syria on Monday, deploying deep inside Kurdish-held territory south of the Turkish frontier less than 24 hours after Washington announced a full withdrawal.
Washington's Kurdish former allies said they invited in the government troops as an emergency step to help fend off an assault by Turkey, launched last week after President Donald Trump moved his troops aside in what the Kurds call a betrayal.
Washington's decision to abandon a policy it had pursued for five years gives Turkish President Tayyip Erdogan and Russia's Vladimir Putin free hands to shape the battlefield of the world's deadliest ongoing war.
In particular, the Syrian army deployment is a victory for President Bashar al-Assad and his most powerful ally Russia, giving them a foothold in the biggest remaining swathe of the country that had been beyond their grasp.
They will now face Turkish armed forces along a new front line hundreds of miles long.
Syrian state media reported the army entered Manbij, a town near the Turkish border in northeast Syria that had been controlled by a militia allied to U.S.-backed Kurdish forces. Earlier it pushed into Tel Tamer, a town on the strategically important M4 highway that runs east-west around 30 km (19 miles) south of the frontier with Turkey.
State television later showed residents welcoming Syrian forces into the town of Ain Issa, which lies on another part of the highway, hundreds of miles away.
Ain Issa commands the northern approaches to Raqqa, former capital of the Islamic State "caliphate", which Kurdish fighters recaptured from the militants two years ago in one of the biggest victories of a U.S.-led campaign.
Much of the M4 skirts the southern fringe of territory where Turkey aims to set up a "safe zone" inside Syria. Turkey said it had seized part of the highway. An official of the Kurdish-led Syrian Democratic Forces (SDF) said clashes were ongoing.
U.S. STRATEGY DISSOLVES
The swift Syrian government deployments underscored how suddenly the strategy the United States had pursued in Syria for the past five years has dissolved. Washington announced on Sunday it was pulling out its entire force of 1,000 troops which had provided air support, ground assistance and training for Syrian Kurds against Islamic State since 2014.
A U.S. official said on Monday a diplomatic team working to help stabilise territory captured from Islamic State had already pulled out. U.S. troops were still on the ground but early phases of their withdrawal had started, the official said.
Two other U.S. officials have told Reuters the bulk of the U.S. pullout could be completed within days.
Sunday's announcement of the U.S. retreat came just a week after Trump gave what the Kurds consider the go-ahead for Turkey to attack by shifting U.S. troops out of the way.
Thousands of fighters from a Kurdish-led force have died since 2014 battling Islamic State in partnership with the United States, a strategy the Trump administration had continued after inheriting it from his predecessor Barack Obama.
Trump says he aims to extract the United States from "endless" wars in the Middle East.
"After the Americans abandoned the region and gave the green light for the Turkish attack, we were forced to explore another option, which is talks with Damascus and Moscow to find a way out and thwart these Turkish attacks," senior Kurdish official Badran Jia Kurd said. Jia Kurd described the new arrangement with Assad's forces as a "preliminary military agreement", and said political aspects would be discussed later.
It remains to be seen how the Kurds will be treated now that they have invited government troops into their region. Kurdish fighters began carving out autonomous rule in Syria's northeast early in its eight-year-old war, benefiting from diversions of Assad's military to fight rebels and militants elsewhere.
Assad aims to restore his government's authority across all of the country.
Another senior Kurdish politician, Aldar Xelil, called the pact with Damascus "an emergency measure". "The priority now is protecting the border's security from the Turkish danger."
NEW FRONT LINE
Ankara says it aims to defeat the Kurdish YPG militia which it views as a terrorist group because of its links to Kurdish separatists in Turkey. A "safe zone" would be set up in Syria to resettle refugees, 3.6 million of whom are in Turkey.
"We are determined to continue the operation until the end, without paying attention to threats," Erdogan said in a speech during a visit to Azerbaijan. "Our battle will continue until ultimate victory is achieved."
The Turkish Defence Ministry said 560 militants had been "neutralised" since the operation began. Earlier, Erdogan said 500 militants had been killed, 26 surrendered and 24 were wounded so far.
The U.S. exit leaves Turkey and Russia, as well as Iran, Assad's main Middle East ally, as Syria's undisputed foreign power brokers. Ankara and Moscow both predicted they would avoid conflict in Syria, even as the front line between them will now spread across the breadth of the country.
"There are many rumours at the moment. However, especially through the embassy and with the positive approach of Russia in Kobani, it appears there won't be any issues," Erdogan said when asked about the prospect of confrontation with Russia. Kobani, on the Turkish border, is one of the first Kurdish-held cities where reports emerged of possible Syrian government deployment.
Kremlin spokesman Dmitry Peskov dismissed the suggestion that Russia could clash with Turkish forces. "We wouldn't even like to think of that scenario," he said.
The fighting has raised Western concerns that the Kurds would be unable to keep thousands of Islamic State fighters in jail and tens of thousands of their family members in camps.
The region's Kurdish-led administration said 785 Islamic State-affiliated foreigners escaped a camp at Ain Issa over the weekend. The British-based war monitor Syrian Observatory for Human Rights, citing sources in the camp, said the number who escaped was smaller, around 100.
Trump, providing no evidence, tweeted on Monday that the Kurds might be releasing Islamic State prisoners deliberately to lure U.S. troops back. Escaped fighters could be "easily recaptured by Turkey or European Nations from where many came, but they should move quickly," Trump said.
EU countries have threatened to impose sanctions on Turkey over the assault. But at a meeting on Monday they agreed not to impose an embargo. Member countries would instead consider their own restrictions on sales of weapons, a measure likely to be brushed off as trivial, as arms account for just 45 million euros out of more than 150 billion euros in Turkey-EU trade.
Republican and Democratic leaders of the U.S. Congress, who have criticised Trump for abandoning the Kurds, have announced plans for legislation to impose sanctions. Turkey's trade with the United States is a fraction of its trade with Europe.