Those Turkish-supported forces recently took up positions near Manbij, where they clashed with Arab forces allied with the USA -backed Kurdish force.
The Marines' deployment, meant to back local forces in the campaign against the Islamic State group, came just days after another intervention.
Iraqi state TV said about half of western Mosul had been taken.
The fall of Raqqa would be a massive blow for Daesh, while an ongoing assault by Iraqi forces on the group's other main city, Mosul, is also depleting its manpower.
The United States is sending an additional 400 troops to Syria to help prepare for the fight for Raqqa, the declared capital of the Islamic State's self-proclaimed caliphate, US officials said Thursday.
The operations are part of Turkey's months-long incursion into its war-torn neighbor in a push against the Islamic State group but also in an effort to restrict US -backed Syrian Kurdish militias, which Ankara has declared a terrorist organization and which it considers to be linked to its home-grown Kurdish insurgency.
From the start, Turkey's goal with its military incursion into Syria has been to push IS militants back from its border and prevent Kurdish forces from holding contiguous territory from east to west across the border.
"Turkey has always set the Euphrates as a red line", Noah Bonsey, an analyst with the International Crisis Group, said in a telephone interview.
SDF fighters have been on the offensive in the Raqqa area since November and have closed major supply roads used by IS.
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The Turkish-backed forces have yet to field a group large enough to tackle Raqqa and had a hard time in liberating Al Bab, which "was a much smaller problem set", he said.
USA conventional forces could be used in stabilization operations in Syria after the initial local force assault on ISIS's capital of Raqqa, Army Gen. Joseph Votel intimated to Congress Thursday. Then, he told the wire service, "the decision to move in can be made".
The Marines' deployment is considered temporary, so it is not affected by the cap. This is a point of contention for Turkey, which fears that if the YPG gains more power, it could pose a threat or funnel support to Kurdistan Worker's Party militants who are in a violent conflict with the Turkish government. The YPG controls swathes of northern Syria, where it has battled ISIL for several years. We have information that the enemy is moving part of its leadership outside the city, as it is also digging tunnels under the ground.
With fulsome U.S. support, the Kurdish-led SDF will spearhead the offensive in Raqqa - a majority-Arab city.
ISIL is also battling the Russian-backed Syrian army in northern Syria and, in separate campaigns, the Turkish army and Free Syrian Army rebel groups which are backed by Turkey.
A Democratic lawmaker is introducing a bill to block additional USA forces from being sent into Syria after an amphibious task force of Marines landed in Syria for the first time.
Assad dismissed the USA -backed military campaign against Islamic State in Syria as "only a few raids" he said had been conducted locally.
"Fighting has entered the first Aamel district today, there was a CTS attack at dawn, and they ran away, retreated", said Captain Muthanna al Saadi of the CTS 2nd division.