WASHINGTON (AFP) - The Pentagon said Monday (Mar 6, 2017) it has sent additional US troops into northern Syria in a show of strength aimed at deterring rival powers from targeting each other instead of the Islamic State in Iraq and Syria (ISIS) group.

Defence Department spokesman Navy Captain Jeff Davis said the troops have in recent days entered Manbij city, a former ISIS bastion that US-backed forces liberated last year.

"We have brought in some additional forces to be able to do this reassurance and deterrence mission," Davis said, without giving numbers.

America wants to make sure competing powers congregating in and around Manbij remain focused on hunting ISIS- and not attacking each other.

The United States is backing a Kurdish-Arab alliance called the Syrian Democratic Forces (SDF) to fight ISIS, and these fighters pushed the extremists from Manbij last year.

Around the same time, Turkey crossed into northern Syria and joined the anti-ISIS fight, while also working to keep in check the Kurdish fighters, which it views as terrorists.

Turkey has threatened to strike the Syrian Kurdish militia forces if they do not withdraw from Manbij.

Last week, Russian and Syrian regime troops headed to Manbij and are now just outside the city, very close to US forces.

That move might be beneficial for the United States, as it could stop Turkey and Kurdish forces - both of whom are US allies - from fighting there.

"This is obviously a really complicated situation, Davis said.

"We have made visible actions in deploying US forces as part of the coalition in and around Manbij to reassure and deter - that's to deter parties from attacking any other parties other than ISIS itself."

The US military currently has about 500 mostly special-operations troops in northern Syria on a train-and-advise mission to help local forces tackle ISIS.

Davis said commanders have the ability to request extra troops if needed.

Their whereabouts is usually a closely guarded secret, so the sight of US armoured-vehicle convoys rolling through Manbij, many flying large US flags, was remarkable.