Washington (AFP) - Washington is considering arming Syrian Kurdish forces who will join the offensive to retake the Islamic State group's stronghold of Raqa, the US military's top officer said Thursday.

Though the United States has already helped arm Kurdish fighters in Iraq, a similar move in Syria is more contentious as key ally Turkey regards the group as terrorists and allies of PKK separatists fighting within Turkish borders.

"We're in deliberation about exactly what to do with the Syrian Democratic Forces (SDF) right now," General Joe Dunford, chairman of the Joint Chiefs of Staff, told the Senate Armed Services Committee.

The SDF numbers about 30,000 fighters and is made up largely of Kurds, though Syrian Arabs also form a significant component.

While the Pentagon has already provided military equipment to the SDF, it insists these shipments have only gone to the Arab part.

The United States is helping train and advise the SDF, as they are expected to conduct the eventual push to retake Raqa, the de-facto capital of IS's self-declared "caliphate."

"They are our most effective partner on the ground. It's very difficult as you know, managing a relationship between our support for the Syrian Democratic Forces and our Turkish allies," Dunford said.

"We're working very closely with our Turkish allies to come up with the right approach ... and still allay the Turkish concerns about the Kurds' long-term political prospects."

When asked by a lawmaker if arming the Syrian Kurds would make the SDF more effective, Dunford said: "I would agree."

"If we would reinforce the Syrian Democratic Forces' current capabilities, that will increase the prospects of our success in Raqa," he said.

Defense Secretary Ashton Carter said no "specific decision" had been taken on the issue. He did not directly respond when asked if he would support arming the Syrian Kurds.

"I support whatever is required to help them move in the direction of Raqa," Carter said.

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Forces loyal to President Bashar al-Assad's regime were pushed out of Raqa, which lies on the Euphrates River, in 2013, making it the first provincial capital in Syria to fall out of government control.

IS rapidly infiltrated the city, which is strategically located near the Turkish border, and declared a caliphate in 2014.

Ousting IS from the city would be a turning point in the conflict and mark a huge blow to the jihadists.