(CNN) The United States' air artillery strikes in northeastern Syria overnight Wednesday were a forceful response to attempts by pro-regime forces to push into areas held by the coalition's allies. The US has drawn a line in the sand -- indicating that any attempt by Syrian President Bashar al-Assad's regime to recover territory east of the Euphrates River will be met with powerful retaliation.
US officials estimate that more than 100 of approximately 500 pro-regime troops involved were killed. The remainder retreated across the Euphrates. The official Syrian state news agency SANA reported that the overnight attack had hit "tribal fighters."
The Euphrates has become a de facto border between regime-held areas and the 23% of Syrian territory held by the US-backed Syrian Democratic Forces (SDF) -- a mix of Kurdish and Arab militia. They hold ground all the way from oil-rich Deir Ezzor in the east to Raqqa and up to the Turkish border.
The US strikes followed what the coalition said was an "unprovoked attack" by forces allied with Assad against a SDF headquarters where coalition advisers were located. The headquarters was at Khusham, almost five miles east of the Euphrates and adjacent to important oil and gas fields. The area was taken from ISIS by the SDF last September, when a rapid advance south supported by coalition air power brought much of the oil-rich countryside east of the Euphrates under SDF control.
The US military assesses that Russian contractors, that are believed to be funded by the Russian government, participated in the assault on US and SDF forces on Wednesday, according to two US defense officials. One official says the assessment comes from monitoring of communications in the area and from drone surveillance.
Col. Thomas F. Veale said Wednesday's attack by pro-regime forces was large-scale, involving battle tanks, rocket launchers and artillery. It was more than a probing operation. A US official said that "20 to 30 artillery and tank rounds landed within 500 meters" of the SDF headquarters. A statement from the US coalition said that "Syrian pro-regime forces moved in an approximately battalion-sized dismounted formation."
US attacks on pro-regime forces in Syria are rare, and the size of this action was unprecedented. Last year, US airstrikes took out small militia columns advancing within a "de-confliction" zone toward a coalition-backed base at al-Tanf on the border with Iraq.
And a US fighter shot down a Syrian plane that had attacked SDF forces near Raqqa last June.
With the defeat of much of the ISIS presence in eastern Syria, it was perhaps only a matter of time before the regime and its allies focused on recovering land to the east of the Euphrates. However, the US has made it clear that it does not intend to leave the area. US officials say that the continued presence of coalition advisers in support of the SDF is essential to prevent the re-emergence of ISIS, and that there may be two or three months more fighting against ISIS remnants in remote areas close to the border with Iraq.
The larger picture is that for the US and pro-western groups such as the Kurdish YPG, those parts of northern Syria beyond the regime's control are a powerful negotiating tool when the future of Syria comes to be discussed, not least because they contain much of Syria's oil. Kurdish politicians have spoken of a confederation that would give them autonomy in northern Syria, but so far they have been excluded from peace talks at Turkey's insistence.
The US has little influence elsewhere in the country, and the conflict has been dominated since late 2015 by Russian air power -- which turned the tide in favor of the regime. But US advisers are also operating in a complex and very large area, with many competing forces. Turkey has declared its intent to drive the Kurdish YPG militia out of parts of northern Syria where the US has a military presence. Its troops have already entered the Kurdish enclave of Afrin, though the US has no military presence in that particular area. There are radical Iraqi and Iranian Shia militia fighting for the regime. And ISIS still retains a presence in some areas -- especially close to the Iraqi border.
Additionally, sources on the ground say Russian contractors and troops are present in Deir Ezzor, albeit in small numbers. One US official told CNN that the involvement of Iranian-backed forces and Russian contractors in Wednesday's attack could not be ruled out.
Assad has vowed to recover every inch of Syria from those he calls "terrorists" -- but the US has repeatedly indicated that it's not about to give up its alliance with the SDF. In a statement released Thursday, the coalition reiterated that the "Syrian Democratic Forces have been instrumental in accelerated successes against Daesh."
Much to the fury of both the Turkish and Syrian governments, the US recently announced the beginning of training for an SDF border force to patrol the Syrian side of the frontier. It also continues to supply the SDF with weapons.