Russian-backed Syrian troops on Saturday pushed into Raqqa province, home to the self-declared capital of the "Islamic State" (IS). At the same time, fighters backed by the United States advanced into the province farther to the north.

Backed by Russian air-power, the Syrian troops are now 50 miles (80 kilometers) from the provincial capital, the city of Raqqa, and closing in on Tabqa, which is home to the country's largest dam.

Raqqa lies just 30 miles downstream from the dam. Liberating the city from IS control is also one of the goals of the US-backed Syrian Democratic Force, or SDF. The SDF's other main goal is to seal off the Syrian border with Turkey in order to prevent militants, supplies and food from reaching IS.

It's unclear whether the Russian-backed assault is being done in tandem with the US effort or contrary to it. But Rami Abdurrahman, director of the Syrian Observatory for Human Rights, said it appeared as though the respective US and Russian-backed troops were acting in concert.

"It seems there has been an undeclared coordination between Washington and Moscow," he said.

US Secretary of State John Kerry spoke by telephone Saturday with his Russian counterpart about Syria, according to the Russian foreign ministry.

Russia's priority

The "main priority" for Moscow is "more direct, effective and forceful measures" against IS and the Nusra Front, according to Russia's Foreign Minister Sergey Lavrov. The US-led Western alliance opposes targeting the jihadist Nusra because of their alliances with so-called "moderate" rebels.

At least 26 jihadists and nine government troops and their militia were killed in the army's initial advance, according to the British-based Observatory, which relies on reports from medics and activists on the ground in Syria.

When IS fighters overran Tabqa in 2014 they seized a garrison and an airbase, and proceeded to summarily execute 160 captured regime troops.

The US-backed SDF controls much of northeastern Syria along the border with Turkey.

The US military said the SDF's push into the Manbij pocket has netted more than 100 square kilometres of territory from IS this past week. IS responded by attacking two towns further west that are held by non-jihadist rebels in a bid to strengthen its access to the Turkish border.

But the US army has dropped ammunition to the rebels defending the town of Marea in the hopes they can protect the town from IS militants.

bik/nm (AP, AFP)