The Pentagon is not coordinating militarily with Russia in northern Syria, even though US- and Moscow-backed forces are drawing closer together as they make gains against the Islamic State group, a US official said Monday.

Syrian regime forces supported by Russian air power and a US-backed, Kurdish-led alliance are conducting separate offensives against the IS group across a broad area to the west of Raqa, the jihadists' de facto Syria capital.

"In terms of direct coordination of activities on the ground, that is not happening," Pentagon press secretary Peter Cook said.

Russian and US military officials speak regularly to ensure their warplanes are not at risk of bumping into each other over Syria, but that is the extent of communications in the war-torn country, Cook added.

"We don't see (coordination) as an issue right now. And if it becomes one, it's certainly something we'll be prepared to address," Cook said.

According to the Syrian Observatory for Human Rights, Russian-backed government fighters are now within 24 kilometers (15 miles) of Lake Assad, the key reservoir in the Euphrates Valley contained by the Tabqa Dam, which is about 50 kilometers (30 miles) west of Raqa.

At the same time, members of a Kurdish-led alliance called the Syrian Democratic Forces (SDF) are now about 60 kilometers (40 miles) north of Raqa, though their immediate focus is on another city called Manbij that is viewed as a key transit point for IS fighters.

Syria's conflict has evolved into a complex war involving foreign powers since it started in 2011 with the brutal repression of anti-government protests. Russia entered the war in support of President Bashar al-Assad last fall.

Peace talks to end the five-year conflict -- which has killed more than 280,000 people and displaced millions -- have stalled and a related ceasefire is in tatters.