Amer al-Halloush, a member of the political branch of a Kurdish-Arab fighting force known as the Syrian Democratic Forces, heads out following a meeting of delegates from Kurdish, Arab, Assyrian and other parties in the town of Rmeilan, Syria. (Delil Souleiman/AFP/Getty Images)

Syrian Kurds unilaterally declared the creation of a federal region in northeastern Syria on Thursday, raising fears of an accelerating disintegration of Syria along ethnic and sectarian lines and complicating efforts to find a peaceful solution to the Syrian crisis.

The declaration defied warnings from Turkey, the United States and the Syrian government that any such move risks further destabilizing the already war-ravaged country, could trigger new conflicts and would not win international support.

It came at the end of a two-day meeting in the northeastern Syrian town of Rmeilan organized by the main U.S. ally in Syria in the fight against the Islamic State, the Democratic Union Party (PYD), which is the dominant political party in the Kurdish areas of Syria.

Supplies of U.S. weapons and extensive U.S. air support have helped the PYD’s military wing, known as the YPG, inflict a string of battlefield defeats on the Islamic State over the past year, driving the militants back from large swaths of territory and also expanding the territories under Kurdish control.

[How the battle against the Islamic State is redrawing the map of the Mideast]

The United States has stressed, however, that it does not support the creation of any kind of new region in Syria by any group, Kurdish or otherwise. “We’ve been very clear that we won’t recognize any kind of autonomous or self-rule, semiautonomous zones in Syria,” State Department spokesman Mark Toner said this week.

In a strongly worded statement, Syria’s government also warned that it would regard as “terrorists” any party that attempted “to undermine the territorial integrity of Syria and the unity of its people,” according to the official Syrian news agency SANA.

“That includes those who gathered” in Rmeilan, SANA added.

The federalism declaration is not intended to detach the northeastern Kurdish region from Syria, only to bring a measure of self-rule, said PYD leader Saleh Muslim, speaking on the sidelines of a conference on the region’s future in the Iraqi Kurdistan city of Sulaymaniyah.

However, he said, it is impossible for Syria to “go back to the old Syria. It is something to be changed. Any kind of centralized Syria is unacceptable.”

In some ways the declaration won’t change much. The PYD had already unilaterally declared the creation of three self-governing “cantons” within a vaguely defined Kurdish area known as Rojava. The self-proclaimed federal region will be known as Rojava-Northern Syria and will link the three cantons — Jazeera, Kobane and Afrin.

[What the ruins of Kobane tell us about the destruction of Syria]

But the self-proclaimed new region includes some areas that are neither Kurdish nor under Kurdish control, such as a stretch of territory between the city of Aleppo and the border town of Azaz that is almost entirely Arab. It is fiercely contested by Syrian rebels, the government, the Kurds and the Islamic State, and both Russia and the United States have conducted airstrikes in the area on behalf of their respective allies.

Turkey has warned that a Kurdish takeover of the Azaz area would be a “red line,” and it has also repeatedly said that it will not tolerate any push for autonomy by the Kurds of northern Syria, whose territories border those of Turkey’s own restive Kurds.

The Turkish government had no immediate response to the declaration. However, a Turkish official, speaking on the condition of anonymity to discuss the sensitive issue, said, “We reject any moves that would compromise Syria’s national unity and consider the territorial integrity of Syria as essential.”

There is also a risk that the declaration will deepen Arab-Kurdish tensions within the areas already under Kurdish control, undermining U.S. efforts to encourage the recruitment of more Arabs into Kurdish ranks for the fight against the Islamic State.

A former Arab schoolteacher, Mansour Salloum, from the mostly Arab town of Tal Abyad in Syria’s Raqqa province, was named the president — in an apparent attempt to endow the new entity with an air of inclusiveness.

But the Syrian opposition has rejected the Kurdish bid for autonomy, and Arab leaders from the area also said they would not back it.

“This is a project to divide Syria, and we will not support it,” said Nawaf al-Bashir, a tribal leader who lives in Turkey and has volunteered fighters to support the U.S.-led campaign against the Islamic State. “We think this announcement is just blowing bubbles,” he added, citing the widespread opposition to the declaration.

Zakaria Zakaria in Istanbul contributed to this report.

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