Airstrikes by the U.S-led anti-terror coalition killed at least 16 people in Syria's northern province of al-Raqqa, a monitor group reported on Wednesday.

The airstrikes marked the first civilian killing by the coalition since the beginning of a new campaign supported by the coalition and led on ground by the Kurdish-led Syrian Democratic Forces (SDF) rebels against the Islamic State (IS) capital in al-Raqqa, according to the Syrian Observatory for Human Rights.

The SDF, a Kurdish opposition alliance that includes Arabs, Assyrians, and Turkmen fighters, announced Sunday the beginning of a military campaign against the IS "capital" of al-Raqqa.

"We, in the general command of the Syrian Democratic Forces, are breaking the good news to you about the beginning of our major military campaign to liberate the city of al-Raqqa and its countryside from the clutches of the forces of darkness represented by Daesh (IS)," an SDF statement read.

The military campaign, dubbed "The Wrath of Euphrates," started on Saturday evening in cooperation and coordination with the U.S.-led anti-terror coalition, it added.

The SDF, meanwhile, urged the regional and international powers that have been affected by the IS to take part in the "honor to eliminate the core of the international terrorism" by providing all kinds of support for the SDF.

The Observatory said 30,000 fighters with the SDF were taking part in the offensive on al-Raqqa, adding that the SDF captured 10 villages and farmlands in the northern countryside of al-Raqqa.

The UK-based watchdog group, which says it relies on a network of activists on ground, said the initial aim of the battle is to isolate the city of al-Raqqa from its northern and eastern countryside, as a prelude to strip the IS of the city itself.

The Syrian government has yet to comment on the SDF battle, but local analysts played down the recently-declaring offensive, saying it's no more than a media campaign.