BEIRUT (AP) — U.S.-led coalition aircraft struck two locations in northern Syria on Saturday, killing at least 21 people, including a woman and her six children who were on a boat fleeing clashes between the Islamic State group and U.S.-backed fighters, activist groups said.

Activist groups that operate in IS-held territory and are opposed to the militants have reported increasing numbers of civilian deaths from U.S.-led airstrikes in recent weeks. The coalition is providing support to a Syrian Kurdish-led force that is on the offensive against the extremist group.

Thousands of people in areas held by IS are trying to flee the violence as preparations are underway to capture the northern city of Raqqa, the extremists' de facto capital. Activists and residents have said the militants are forcing civilians to stay in order to use them as human shields once the offensive on Raqqa begins.

Leaflets dropped by coalition warplanes in recent weeks gave confusing directions — one suggests areas closer to the Euphrates River are safer, but another warns that boats crossing the river will be struck.

Raqqa is Being Slaughtered Silently, an activist group opposed to IS, said a boat carrying about 40 people was hit as it was crossing the Euphrates River in Raqqa province. It said the bodies of a woman and her six children were recovered, while the others are still missing. The Sound and Picture group, which tracks killings and other abuses in IS-held territory, reported the same casualty estimates. Journalists have virtually no access to areas held by IS, which has captured and killed a number of reporters.

The strike occurred in the Shuaib al-Zeker area, near where the Kurdish-led Syrian Democratic Forces are battling IS.

Earlier Saturday, activists and state media said an airstrike by the U.S.-led coalition on the northern IS-held village of Hneida killed at least 14 civilians, including children. The Britain-based Syrian Observatory for Human Rights said 15 people, including four children, were killed in the airstrike. The Sound and Picture group the airstrike hit an internet cafe, killing 14 people.