Beirut: A US-backed Syrian rebel alliance on Saturday announced a fresh offensive against Islamic State in the northeast province of Hasaka, a day after the United States said it would send special forces to advise insurgents fighting the jihadists. 

It was the first declared operation by the Democratic Forces of Syria, which joins together a US-backed Kurdish militia and several Syrian Arab rebel groups, since it announced its formation earlier this month. 

"We announce today the start of the first step in our military operations," a spokesman for the alliance's general command said in Arabic in a video statement posted on YouTube. 

World powers and regional rivals are convening in Vienna to seek a solution to the four-year conflict in Syria that has escalated since Russia intervened a month ago with an intense air campaign. 

The video showed several dozen men in fatigues standing outdoors with yellow flags and banners carrying the name of the Democratic Forces of Syria in Arabic and Kurdish. 

The Kurdish YPG has been working closely with the U.S.-led anti-IS coalition since early this year, and has to date proved Washington's most effective partner on the ground against IS in Syria. 

The United States, which announced on Friday it was to station its first ground troops in Syria for the war on IS, has dropped ammunition to rebels in the north of Syria but without specifying which group the aid was intended for. 

"With the participation of all the factions... and with support and coordination with the planes of the international coalition to fight Daesh, we announce the start of the liberation of the southern countryside of Hasaka," the spokesman said, using an Arabic acronym for Islamic State. 

The campaign would "continue until all occupied areas in Hasaka are freed from Daesh," he said, urging local residents to stay away from IS-controlled areas of Hasaka. 

The statement was also reported by the Britain-based Syrian Observatory for Human Rights, which monitors the war. 

Hasaka province borders Iraq and Islamic State territory there, a crucial stronghold for the group.  The YPG had pushed towards the border in previous fighting this year. 

The Democratic Forces of Syria includes a number of Arab rebel groups. 

One of those groups, the Raqqa Revolutionaries Front, on Thursday declared an imminent offensive against Islamic State in its Syrian stronghold of Raqqa province, which borders Hasaka. 

The Raqqa Front told Reuters earlier in October that US weapons were on their way.