At least 16 civilians were killed in bombing raids early Wednesday by the US-led coalition near the Islamic State group's Syrian bastion Raqqa, a monitor said.

The Syrian Observatory for Human Rights (SOHR) said the toll included a woman and her five children, as well as three couples.

"The coalition strikes hit al-Baruda, a village about 15km west of Raqqa city," said Observatory head Rami Abdel Rahman.

"Most of those killed had fled eastern parts of the province of Homs," he added.

The US-led coalition is providing air cover for a major offensive to capture Raqqa city, the heart of IS territory in Syria.

As of Wednesday, the US-backed Syrian Democratic Forces (SDF) were only three kilometres (two miles) from Raqqa at their closest point to the east.

The strikes on al-Baruda come after the Observatory reported the highest monthly civilian death toll for the coalition since it began bombing Syria on 23 September 2014.

Between 23 April and 23 May of this year, coalition strikes killed a total of 225 civilians in Syria, the Britain-based Observatory said.

Earlier this month, the US military said that coalition air strikes in Iraq and Syria had "unintentionally" killed a total of 352 civilians since 2014.

More than 320,000 people have been killed and millions more displaced since Syria's conflict broke out in March 2011.