U.S.-led air strikes on Syria killed a total of 225 civilians over the past month, a monitor said Tuesday, the highest 30-day toll since the campaign began in 2014.

The Syrian Observatory for Human Rights said the civilian dead between April 23 and May 23 included 44 children and 36 women. The U.S.-led air campaign against the Daesh terrorist group in Syria began on September 23, 2014. "The past month of operations is the highest civilian toll since the coalition began bombing Syria," Observatory head Rami Abdel Rahman told AFP. "There has been a very big escalation."

The past month's deaths brought the overall civilian toll from the coalition campaign to 1,481, among them 319 children, the Britain-based monitoring group said.

The U.S. military said in May that coalition air strikes in Syria and Iraq had "unintentionally" killed 352 civilians since operations against Daesh began. The strikes between April 23 and May 23 also killed 122 Daesh militants and eight fighters loyal to the Syrian regime, the Observatory said.

The U.S.-led coalition has been bombing Daesh in Syria since 2014 and is backing an offensive to defeat the group in Raqqa city, the de facto heart of the group's so-called "Islamic caliphate."

The regime's news agency SANA also reported the air raid, accusing the U.S.-led coalition of inflicting "dozens" of casualties and almost completely destroying the school site.

In March, the coalition said its raids Syria and in Iraq and unintentionally killed at least 220 civilians. But other monitors say the number is much higher. More than 320,000 people have been killed and millions more displaced since Syria's conflict erupted in March 2011 with protests against Bashar Al-Assad.

Meanwhile, an explosion in the central Syrian city of Homs yesterday killed at least four people and wounded more than two dozen while Assad forces "destroyed" a truck bomb near the Damascus airport, Syria's state media reported. The blast in Homs came two days after opposition fighters left al-Waer, the city's last opposition-held neighborhood, bringing the entire city back under regime control for the first time in more than five years.