BEIRUT - A U.S.-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 U.S.-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.(Reuters)