U.S.-backed Syrian Kurdish-Arab forces have announced the start of a campaign to retake the Islamic State (IS) militant group's de facto capital of Raqqa, Syria.
The Syria Democratic Forces (SDF), a multi-ethnic opposition alliance led by the Kurdish People's Protection Units (YPG), made the announcement in the Syrian town of Ain Issa, north of Raqqa.
"The general command of the Syria Democratic Forces announces the blessed start of its major military campaign to liberate the city of Raqqa," Jehan Sheikh Amad, an SDF spokeswoman, said on November 6.
The SDF called on Raqqa's civilians to avoid areas where IS militants are present and to go to "liberated territory."
There was no immediate reaction from Syrian President Bashar al-Assad's government about the SDF announcement.
The announcement comes as Iraqi government forces -- backed by U.S.-led coalition air strikes, Iraqi Kurdish Peshmerga, Sunni Arab tribal fighters, and Iranian-backed Shi'ite militia -- battle to retake the northern Iraqi city of Mosul from IS extremists.
The United States considers the YPG militias the most effective force against IS militants in Syria.
Unease In Ankara
But Turkey is dismayed at the prominent role played in the SDF by the Kurdish YPG militia. It also accuses those Syrian Kurdish fighters of having links with Turkey's outlawed Kurdistan Workers Party (PKK) -- which has been designated by both Washington and Ankara as a terrorist organization that has waged a three-decade insurgency within Turkey.
Turkey also has launched an offensive into northern Syria where Turkish forces have clashed with SDF fighters who have seized large swaths of territory from IS along the Syria-Turkey border.
Brett McGurk, U.S. President Barack Obama's envoy to the U.S.-led coalition that is fighting against the IS militants, said in Amman on November 6 that U.S. military officials are "in close contact" with Turkish military leaders about the operation.
He also said that the United States is providing close air support for the attack on Raqqa.
British Defense Secretary Michael Fallon, meanwhile, said the Royal Air Force is providing aerial surveillance to help the offensive and "will support the Raqqa operation as it develops."
McGurk told reporters in the Jordanian capital that Washington wants the offensive against Raqqa "to be as coordinated as possible, recognizing that there will be a mix of forces on the field and that many of those forces, of course, do not see eye-to-eye -- but they do share a very common and still very lethal enemy."
McGurk also announced that General Joseph Dunford, Chairman of the U.S. Joint Chiefs of Staff, arrived in Ankara on November 6 for a previously unannounced visit that includes talks with his counterpart in Turkey's army, General Hulusi Akar.
Turkish officials, including President Recep Tayyip Erdogan, have said that they will not accept a role for Syrian Kurds in the liberation of the city of Raqqa itself.
Turkey's defense minister last week suggested that instead of the Syrian Kurds, a Turkish-backed force could present an "alternative."
In a speech on November 6, Erdogan did not comment on the SDF announcement but said allied Syrian opposition fighters were fast approaching the Syrian town of Al-Bab, the last stronghold of the IS group in Aleppo Province.
Breaking Islamic State's 'Barbaric Grip'
U.S. Defense Secretary Ash Carter on November 6 welcomed the announcement on the SDF operation to liberate Raqqa from what he called the "barbaric grip" of the so-called Islamic State.
Carter said. "The effort to isolate, and ultimately liberate, Raqqa marks the next step in our coalition campaign plan. As in Mosul, the fight will not be easy and there is hard work ahead, but it is necessary to end the fiction" of an Islamic State caliphate and to "disrupt the group's ability to carry out terror attacks against the United States, our allies, and our partners."
On November 2, Carter had said that YPG militia fighters would be part of the force charged with surrounding and isolating IS militants in Raqqa.
But U.S. officials say Arab forces in the SDF were expected to go into the city of Raqqa itself in order to avoid potential ethnic conflicts.
A similar strategy has been proposed in the operation to retake Mosul from IS militants to avoid igniting ethnic and sectarian conflicts.
Carter's November 2 comments suggested that Washington may try to involve Turkey in the final takeover of the city after Kurdish YPG forces have helped to surround and isolate it.
An attack on Raqqa has long been expected, with Carter saying on October 25 that the battle to retake it would "overlap" with the campaign in Mosul.
The top U.S. military commander in Iraq, Army Lieutenant General Stephen Townsend, said in October that the U.S.-led coalition fighting IS militants wanted to move urgently to isolate Raqqa because of concerns about the group using the city as a base to plan and launch attacks against targets abroad.