A newly formed U.S.-backed Syrian rebel alliance on Saturday launched an 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.

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. […]