For years, the Syrian government and U.S.-backed Kurdish forces shared control of Hasakah, a city in Syria's northeast. But last week, the Kurdish militia known as the People's Protection Units, or YPG, launched an assault on government-controlled areas of the city, activists said — marking a rift between onetime allies and further blurring the lines between the warring sides in Syria's bloody internecine civil war.
More than 50 people had been killed in the clashes, including fighters and civilians. It is the most violent confrontation between pro-government and Kurdish forces since the start of the civil war, said Rami Abdul Rahman, head of the pro-opposition Syrian Observatory for Human Rights.
The Kurds have played both sides of the Syrian conflict, forging tactical alliances with both the Syrian government of President Bashar Assad as well as the rebels fighting to end his rule. They have also worked with other countries involved in the civil war, and emerged as the biggest recipient of U.S. largesse, which has given them weaponry as well as logistical weapons support.
Ever since the opposition took over large swaths of the country's resource-rich northern areas, a number of rebel factions have battled the YPG in areas around Aleppo. Many accuse the YPG of seeking to consolidate areas extending from Syria's northeastern tip to Afrin, roughly 250 miles to the west.
In July, the YPG assisted pro-government forces in establishing control over the Castello road, a strategic highway that had linked rebel-held areas of Aleppo city to the Turkish border.
But on Tuesday, a dispute between the Asayesh, a Kurdish internal security force, and a pro-government militia escalated into a full-blown drive by the Kurds to consolidate their control of the city.
Then, on Thursday, Syrian warplanes launched unprecedented airstrikes on Kurdish positions in the area. Hundreds fled the fighting.
The Pentagon said U.S. special operations forces were caught in the middle when they, acting as advisors in the region, were nearly hit Friday with a Syrian airstrike.
In response, the U.S. scrambled two F-22 stealth fighter jets to monitor the area around Hasakah and chase away any Syrian jets that entered the airspace. U.S. officials, who were not authorized to speak publicly on the matter, said Syrian jets have not flown near Hasakah since Friday and that American warplanes continue to police the region, providing de facto aerial protection for Kurdish forces that U.S. officials nevertheless insisted was not a formal no-fly zone.
The Kurdish security force, the Asayesh, issued leaflets telling government troops they had to "lay down their arms… or consider [themselves] dead," according to images uploaded to social media.
Russia brokered a brief ceasefire between the two sides Sunday — but hours later, shortly after midnight Monday, Kurdish forces relaunched their offensive. They moved to encircle government positions in what was seen as a final bid to oust Damascus' remaining presence from the city.
Kurdish forces had now completely surrounded government troops that had been forced to retreat from the southern neighborhoods of Nashwa and Ghweiran, said Fahed Fataah, a local resident.
Kurdish fighters took down images of Assad and Syrian flags that adorned government buildings and replaced them with the YPG's banner, activists said. They imposed a curfew on neighborhoods under their control.
A pro-government Facebook community, "The Men of the Syrian Army in Hasakah," corroborated Fataah's account, but added that Russia was still mediating negotiations despite the ceasefire's breakdown.
But a ceasefire might prove elusive. "The Asayesh will not back down no matter what concessions the regime offers," Abu Araaj, a spokesperson for a YPG-allied Arab coalition called the Army of the Revolutionaries, said Monday. He gave a nom de guerre for reasons of security.
He added that Arab units were not involved in the offensive but would intervene if required.
The escalation between the Kurds and the Syrian government came as two of Damascus' key allies appeared to be in disagreement. Iran chastised Russia for revealing that Russian Tupolev-22M3 strategic bombers and Sukhoi-34 fighters had used the Shahid Nojeh airbase near the city of Hamedan to mount bombing runs in Syria.
Iranian Defense Minister Hossein Deghan criticized Russia for exhibiting a "show-off and ungentlemanly [attitude] in this field," the BBC reported.
The operation is now over and Russian units are no longer at the base, Iran Foreign Ministry spokesman Bahram Qasemi said Monday, according to Iran's Press TV. But Russian Defense Ministry spokesman Igor Konashenkov suggested that there would be future deployments of Russian warplanes to Iran, "based on mutual agreements on fighting terrorism and depending on developments in Syria," Russia Today reported.
Meanwhile, the Kurdish offensive on Hasakah, seen as a further bid to create a Kurdish autonomous entity on Syrian soil, has pushed Turkey to soften its stance toward Assad.
Turkey views the YPG as a proxy for the Kurdistan Workers' Party, with whom it has waged a decades-long insurgency war and counts as a terrorist force on a par with Islamic State. It fears that a Syrian Kurdish presence on its southern border could give rise to a fresh PKK campaign for an independent Kurdish state -- a concern it now shares with Damascus.
The clashes pushed Turkish Prime Minister Binali Yildirim to say that the Syrian leader could remain in a transitional role Saturday. He also said that Incirlik airbase, used by the U.S.-led coalition to launch sorties against Islamic State, could be opened to Russia as well.
The offensive has even led to a rare point of agreement between Assad and the opposition: Both Damascus and the rebels have insisted they aim to maintain Syria's territorial integrity and are vehemently against what they describe as "secessionist projects."
Washington's aerial support for the Kurds, which has effectively grounded Assad's warplanes over Hasakah and given the YPG an immense advantage against government troops, is likely to further alienate Turkey.
Nonetheless, the U.S. defended its aerial patrols over Hasakah by maintaining it will not tolerate any threat that's posed to the more than 300 U.S. special forces commandos, many of whom are stationed in northeast Syria.
Pentagon spokesman Peter Cook told reporters Monday that the U.S. military informed the Russian military where the troops were located through an emergency communications channel set up to avoid air accidents. The Russians have passed the message to the Assad government, he said.
"We would continue to advise the Syrian regime to steer clear of those areas," Cook said. "As our forces move through Syria and continue their partnered operations, we will do what we need to do to protect our forces."