Story highlights US-backed fighters are preparing to move in the coming weeks to assault the city of Raqqa

Raqqa is ISIS' self-declared capital

(CNN) US Marines have arrived in northern Syria with artillery to support US-backed local forces fighting there, two US officials told CNN.

The US-backed fighters are preparing to move in the coming weeks to assault the city of Raqqa, ISIS' self-declared capital, according to the officials. The Pentagon and the Marine Corps have declined to confirm the deployment because of security concerns in the region. They have also declined to specify the exact location of the forces or how many are there.

The Washington Post was the first to report the deployment of the Marines.

Photos: Life inside Raqqa In this photo from November 29, 2015, provided to CNN by the activist group Raqqa is Being Slaughtered Silently, residents assess the damage to a building in the northern Syrian city -- ISIS's headquarters -- which has been the target of French airstrikes in recent weeks. Hide Caption 1 of 8 Photos: Life inside Raqqa In this photo from November 6, 2015, an ISIS fighter walks along a street in Raqqa with his 3 wives walking behind him, according to RBSS. Hide Caption 2 of 8 Photos: Life inside Raqqa A child looks at a stand selling military fatigues in Raqqa on October 1, 2013. Many in Raqqa say they don't want to live under ISIS but have no choice. Hide Caption 3 of 8 Photos: Life inside Raqqa Men look at a large black jihadist flag in Raqqa on September 28, 2013. In Raqqa today, school is banned -- and even small pleasures, like chocolate, are an unaffordable luxury because many cannot work. Hide Caption 4 of 8 Photos: Life inside Raqqa A Syrian man mourns the deaths of six of his siblings, killed in a bomb attack during fighting between rebel fighters and Syrian government forces in Raqqa on August 10, 2013. Hide Caption 5 of 8 Photos: Life inside Raqqa A man carries two children away from the scene of an explosion in Raqqa on August 7, 2013. Hide Caption 6 of 8 Photos: Life inside Raqqa In this undated photo, provided to CNN by RBSS, you can see normal life in Raqqa -- once one of Syria's most liberal cities -- before the start of the civil war. Hide Caption 7 of 8 Photos: Life inside Raqqa The streets of Raqqa before it was under ISIS control, in an undated photo provided to CNN by RBSS. Hide Caption 8 of 8

The deployment does not come as a surprise. Military commanders have discussed for weeks the possibility of putting artillery forces into the area, with the goal of accelerating the capabilities of the US-backed Arab and Kurdish forces there. A similar deployment last year near Mosul, Iraq involved several hundred Marines equipped with artillery guns that fire shells to provide covering fire for advancing forces.

Because Marines were already deployed to the region, the movement into Syria did not have to be specifically approved by President Donald Trump or Defense Secretary James Mattis -- but both the White House and Pentagon were aware of the plan, officials said.

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