Following last week’s threats by Washington to attack Syrian government war planes bombing US-backed Kurdish forces in the northern Syrian town of Hasakeh, Turkey has launched artillery barrages against both ISIS fighters and Kurdish militia in and near the Syrian border town of Jarablus.

Turkey claims shells that fell Monday on its border towns of Karkamis and Kilis were fired by ISIS forces occupying Jarablus.

In addition to retaliatory shelling, Ankara is assembling a force of some 1,500 Syrian “rebels” in the Turkish city of Gaziantep, which was hit by the suicide bombing of a Kurdish wedding Saturday that killed at least 54 people and wounded dozens more. Turkey has blamed ISIS for the atrocity.

The state-run Anadolu Agency reported that Turkey had increased security on its border opposite Jarablus, deploying tanks and armored personnel carriers.

The “rebel” force in Gaziantep is expected to cross into Syria with the aim of breaking ISIS control over Jarablus and at the same time preventing the Kurdish-led, US-backed Syrian Democratic Forces from filling the resulting power vacuum.

This latest explosive turn in the tangle of shifting alliances and conflicts among the global powers intervening in Syria threatens to bring Turkey into direct conflict with Washington’s chief proxy force in northern Syria, the Kurdish People’s Protection Units, or YPG, which is the backbone of the Syrian Democratic Forces (SDF).

Earlier this month, the SDF, backed by intensive and deadly US air support, drove ISIS out of the strategic town of Manbij. This alarmed the government of Turkish President Recep Tayyip Erdogan, which is waging a brutal war against the Kurdish separatist Kurdistan Workers Movement (PKK) within Turkey. It fears that the YPG victory in Manbij will further consolidate a de facto Kurdish enclave in northern Syria that is being set up with the tacit support of the United States.

Turkey and the US, NATO allies, are increasingly working at cross purposes in Syria, further frustrating Washington’s central aim in the horrific war it has inflicted on the country, the removal of the pro-Iranian and pro-Russian regime of Syrian President Bashar al-Assad and the installation of a US puppet government. The looming confrontation in Jarablus follows Turkey’s accusations of US complicity in the failed military coup of July 15, Erdogan’s turn to a rapprochement with Russia and Iran, and Ankara’s softening of its opposition to Assad.

It also coincides with the visit today of US Vice President Joseph Biden to meet with top Turkish officials under conditions that were already fraught with tension.

The New York Times on Tuesday quoted Nasswer Haj Mansour, an SDF official on the Syrian side of the border, as saying the forces gathering in Turkey included “terrorists” as well as Turkish Special Forces. A statement from the SDF declared that “we are prepared to defend the country against any plans for a direct or indirect occupation.”

Abdel-Sattar al-Jader, a rebel commander aligned with the SDF, was killed late Monday shortly after broadcasting a statement proclaiming the formation of the “Jarablus Military Council” and pledging to protect civilians in the town from Turkish “aggression.” Al-Jader was shot by unidentified gunmen. The Military Council subsequently blamed his murder on Turkish security agents. Haj Mansour said two suspects were in custody but would not reveal their identities.

There is an evident convergence between the expanding attack by Turkey on Kurdish forces in Syria and the more aggressive posture of the Assad regime toward the YPG and SDF. Last week, after six days of fighting between the YPG and Syrian troops and pro-government militia in Hasakeh, which has been split between the two camps since the early days of the Syrian civil war, the Syrian Air Force for the first time bombed YPG positions. That move was apparently in response to an attempt by the Kurds to drive the pro-government forces out and take control of the entire city.

Just how explosive and potentially catastrophic the situation in Syria is, and how reckless the policy of Washington, was demonstrated by the US response. Claiming that some of its Special Forces troops embedded with the Kurdish militia—completely illegally—were endangered by the government bombing, the US scrambled jets to confront the Syrian warplanes, setting up a possible military clash with the Russian-backed Syrian forces.

Pentagon spokesmen followed with threats of US retaliation in the event of further bombings. Lt. Gen. Stephen J. Townsend, who on Sunday took over command of US and allied operations in Syria and Iraq, told CNN, “We will defend ourselves if we feel threatened.”

That the increasingly conflicted and complex situation in Syria could quickly escalate into a far greater and more bloody conflagration, possibly involving nuclear exchanges between the US and Russia, is shown by the deployment in Hasakeh on the side of the Syrian government of both Iranian and Hezbollah forces, and the presence of British and French, as well as American, Special Forces within the Kurdish-led SDF.

On Tuesday, Syrian state media and the Kurdish Hawar News Agency both announced the implementation of a cease-fire in Hasakeh, evidently brokered by Russia. However, while the Kurdish statement said government forces had agreed, as part of the cease-fire terms, to withdraw from the town and leave it under the control of the local Kurdish police force, the Syrian statement made no mention of a withdrawal.

There were other indications of a moderation of the animus between the Assad regime and Turkey. On Friday, the Syrian military’s General Command, in an evident concession to Turkey, released a statement referring to the Kurdish Asayesh internal police in Hasakeh as the “military wing of the Kurdistan Workers Party.” Turkey has long pressed Damascus to declare the Syrian Kurdish forces to be an extension of the PKK.

From the other side, Turkish Prime Minister Binali Yildirim, speaking to foreign media Saturday in Istanbul, spelled out a shift in Turkey’s posture toward Assad, saying for the first time that while Assad could not be part of a long-term solution to the crisis in Syria, Ankara was willing to accept a role for him in a transitional government. At the same time, Yildirim stressed that Turkey would intervene more actively in the Syrian crisis and would not permit the country to be divided along ethnic and sectarian lines—an implicit criticism of US policy toward the Syrian Kurds.

Within this explosive mix of great power brigandage and conflicting geo-political interests, which in general is becoming increasingly unfavorable to the realization of Washington’s imperialist aims, the US is preparing to escalate its military violence.

On Monday, the new US commander, General Townsend, said Washington would step up its operations in support of its proxy forces as they prepared offensives to retake Mosul in Iraq and Raqqa in Syria from ISIS. He said the escalation would include intensified air and artillery strikes and increased efforts to equip and train local forces. He left open the possibility of an enlargement of the US troop presence in the two countries.