Soldiers from the Syrian army carry a rocket to fire at ISIS positions in the province of Raqqa, Syria.

Washington is in “close contact” with Ankara over the assault launched by American-backed Kurdish-Arab forces on ISIS’s bastion Raqa in Syria, a senior US official said Sunday.

“We are in close close contact with our Turkish allies and that is why the chairman of joint chiefs is in Ankara today,” Brett McGurk, President Barack Obama’s envoy to the US-led coalition battling the militants, told a news conference in the Jordanian capital Amman.

“We want this 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,” he said of ISIS.

The chairman of the US joint chiefs of staff, General Joseph Dunford, arrived Sunday in Ankara on a previously unannounced visit and was to meet his Turkish counterpart Hulusi Akar, the Turkish army said earlier, without elaborating.

‘Complex environment’

“It is a complex environment in Syria to say the least, but we are constantly in touch with all the different players, and I think in terms of the phasing of the overall Raqqa campaign we have a fairly good understanding of what is to come,” said McGurk.

Kurdish-led Syrian forces backed by the US began an offensive Sunday to liberate ISIS’s de facto capital of Raqqa, clashing with the extremists north of the Syrian city.

The attack ratchets up pressure on the militant group at a critical moment, with its fighters already battling an offensive by Iraqi security forces on their remaining Iraqi stronghold in the northern city of Mosul.

The US-backed Syria Democratic Forces (SDF), an alliance of Kurdish and Arab armed groups, first announced on Sunday that a campaign to retake Raqqa would begin within hours, with US forces providing air cover. Soon afterwards, it said that the operation, called Euphrates Anger, had begun.

“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, told a news conference in the Syrian town of Ain Issa.

The SDF called on Raqqa’s civilians to avoid areas where ISIS militants are present and to go to “liberated territory.”

US and British officials said they would provide air support for the offensive, which was announced at a news conference in Ein Issa, north of Raqqa, by SDF. But it lacked details on how the group dominated by Kurds plans to oust the militants from the city, home to nearly 200,000 mostly Sunni Arabs and an estimated 5,000 ISIS fighters.

‘Fight will not be easy’

Meanwhile, US Defense Secretary Ashton Carter warned on Sunday that the fight to wrest control of Raqqa “will not be easy.”

“The effort to isolate, and ultimately liberate, Raqqa marks the next step in our coalition campaign plan,” Carter said in a statement.

“As in Mosul, the fight will not be easy and there is hard work ahead, but it is necessary to end the fiction of ISIL’s caliphate and disrupt the group’s ability to carry out terror attacks against the United States, our allies and our partners,” he said, using an alternative name for the militant group.

“The international coalition will continue to do what we can to enable local forces in both Iraq and Syria to deliver ISIL the lasting defeat it deserves,” the US defense chief added.

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