A coalition of Syrian Arab and Kurdish forces backed by US-airstrikes continued their advance towards the strategic "Islamic State" (IS)-controlled city of Manbij on Sunday.

The Syrian Democratic Forces (SDF) offensive to seal an IS-held section of the Turkish border and cut off the extremist group's de-facto capital Raqqa hinges on taking Manbij, a city on the west side of the Euphrates that acts as a major transit point for IS fighters and supplies.

The SDF advanced to within eight kilometers of Manbij and captured the Raqqa-Manbij road on Sunday after taking dozens of villages west of the Euphrates in the past days, ANHA news agency, which is close to the Syrian Kurds, reported.

The agency released photos from an SDF hilltop position which claims to show grain silos in Manbij in the distance.

The UK-based Syrian Observatory for Human Rights (SOHR) said SDF forces had advanced to within five kilometers of Manbij.

US Special Forces are embedded with SDF fighters to coordinate and call in dozens of airstrikes on IS positions. SDF has occasionally clashed with Syrian regime forces but on the whole there is a tacit understanding between it and the regime.

US Central Command spokesman Colonel Patrick Ryder said on Saturday that SDF forces had taken 100 square kilometers (40 square miles) of territory since launching the offensive nearly a week ago.

Preparations to take the so-called "Manbij pocket" have been long in the making since SDF forces took the Tishreen dam on the Euphrates River in December.

The operation has been complicated by Turkish objections to fighters belonging to the Syrian Kurdish militia YPG crossing to the west of the Euphrates.

The YPG are considered the best fighters against IS but are affiliated with the Kurdistan Workers' Party (PKK), with which Ankara has waged a three-decade long war and considers a terrorist organization.

The SDF is composed mostly of Syrian Kurdish fighters, but also includes Turkmen, Arab and Assyrian Christian units.

The offensive in Manbij is led mostly by Arab forces due to both Turkish sensitivities and the belief they will be better able to hold territory that is traditionally inhabited by Arabs. US Special Forces have been coordinating the training of Arab units and providing weapons in anticipation of the offensive.

The push to the west of the Euphrates comes some two weeks after a rare visit to northern Syria by US Central Command Commander General Joseph L. Votel, who met with SDF forces to assess their strength. The visit prompted speculation the US-backed forces may be preparing for a large scale offensive.

The SDF also announced some 10 days ago an offensive in the northern countryside of Raqqa, but that offensive appears to have been in part a decoy to keep IS off its feet. The announcement was followed several days later by the Manbij offensive.

Syrian regime offensive

To the southeast of Manbij, Russian airstrikes pounded IS positions as Syrian regime forces and allied militia moved into Raqqa province on Saturday for the first time since August 2014.

The hollowed out Syrian army fighting the Islamic State and myriad rebel groups is backed by Iranian forces, Shiite militia and Lebanese Hezbollah.

Regime forces now stand some 40 kilometers (25 miles) from Tabqa, site of the country's biggest dam along the Euphrates and a gateway to Raqqa.

It was unclear if Russian and Syrian forces had planned the strike into Raqqa and towards the dam separately of the US and SDF offensive, or if they were coordinated.

Separately, Russian and Syrian warplanes hammered rebel positions with nearly 40 airstrikes in and around Aleppo on Sunday.

The airstrikes appear to be aimed at softening up a key rebel held road into the city that provides the only outlet to rebels in the eastern part of the city. Syrian regime and allied forces are trying to take the road to completely encircle rebel-held parts of the city.

Recapturing Aleppo, Syria's largest city and commercial hub before the war, would be a major victory for the Assad regime.

Turkey and Saudi backed rebel factions in the northern Aleppo countryside near the border with Turkey are meanwhile on the verge of collapse following an IS offensive around Azaz and Marea.

Separately on Sunday, Iran shot back at a US State Department report accusing Iran of being a major sponsor of terrorist groups, by among other things backing Hezbollah and the Assad government.