The US military called for an end to clashes between Turkey, Syrian rebels and Kurdish fighters as the NATO ally pushed deeper into Syria, bringing the diverging priorities of the various American allies into stark relief.

The Pentagon said it was not supporting the Turks or the Syrian rebels against the US-backed Kurds, and that the clashes were "unacceptable and they are a source of deep concern."

The US has been providing air cover to aid Turkey's incursion into northern Syria — called Operation Euphrates Shield — in what Turkey said was an effort to move Islamic State militants away from its border with Syria and to halt the expansion of Kurdish-controlled territory.

The problem is that the Kurdish fighters are also US allies, and lead the US-backed Syrian Democratic Forces — a key ally in the fight against the Islamic State.

The US Department of Defense was quick to distance itself Monday morning from the fighting and Turkish airstrikes, which at least one independent monitor said on Sunday killed 40 civilians.

The Turks say the dead are Kurdish fighters. The airstrikes came after Kurdish fighters ambushed two Turkish tanks in an attack that killed at least one Turkish soldier.

"The United States was not involved in these activities, they were not coordinated with US forces, and we do not support them," said Pentagon spokesman Peter Cook. "We regret all loss of life in these reported clashes and have expressed our condolences to Turkey for the apparent loss of a Turkish soldier."

Cook called on the Turks, Kurds and Syrian rebels to "stand down immediately" and "deconflict" in order to focus on defeating the Islamic state

The various warring parties in Syria have different objectives though, and the US is now facing a splintering of what was already a shaky coalition of forces who hold wildly varying priorities.

The past week's conflict arises from the US dependence on the Kurdish-led Syrian Democratic Forces (SDF) as its ground force in the fight against the Islamic State.

The SDF is dominated by the YPG, which is the Syrian wing of the PKK, a Kurdish guerrilla group in Turkey that the US and Turkey both classify as a terrorist organization.

The US has helped the SDF push Islamic State from key Syrian towns like Manbij, but the Turks don't want the Kurds to remain in those towns, for fear the Kurds will try to create their own autonomous enclave. The Turks worry that if the Kurds in Syria achieve quasi-independence, then Turkey's Kurds — against whom it is already fighting a bloody insurgency in its southeast — will want the same.

Turkish Deputy Prime Minister Numan Kurtulmus said on Monday that the goal of Euphrates Shield was to prevent the Kurds from "completing an end-to-end corridor" across Syria.

"If that happens, it means Syria has been divided," he said, according to AFP.

Other Syrian observers criticized the deteriorating situation in northern Syria on social media.