(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.
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.
The Marines deployed from ships in the Persian Gulf region.
This is the second major expansion of US ground forces in northern Syria in days.
The US has also deployed approximately 100 Army Rangers in and around Manbij, Syria. US officials have taken the unusual step of publicly talking about the Ranger deployment and where they are located to protect against them inadvertently coming under fire from forces fighting in the region or Turkish, Russian or Syrian forces. The US troops in Manbij are trying to deter hostilities due to their visible presence, rather than the typical mission of training, advising and assisting local forces.
It is also not clear if the deployment of the Marines and Rangers violates the current cap on US forces in Syria. That cap broadly restricts the US to having more than 500 forces there at any one time. Until now, troops in the area have largely been Special Operations Forces.
The US believes the pressure on ISIS in Raqqa is working. A US official told reporters Wednesday that intelligence indicates some ISIS leadership and operatives continue to try to leave the city. There is also US intelligence that indicates the city is laced with trenches, tunnels, roadside bombs and houses and buildings wired to explode, the official said. If correct, it indicates that the US has likely been able to gather intelligence from both overhead surveillance aircraft and people on the ground.
However, the official also noted that "Raqqa will probably not be the final battle against ISIS," and added that the group still has some personnel dispersed in areas south and east of the city.
ISIS could have as many as 4,000 fighters in Raqqa, according to very rough US estimates, the official said.