Mandatory school mask requirements lowered COVID-19 infection outbreaks among children, according to the US Centers for Disease Control and Prevention (CDC).

The CDC analysed 520 US counties and found that paediatric cases rose sharply in places where school mask requirements were not made mandatory, The Washington Post reported.

Schools without mask requirements were 3.5 times more likely to experience an outbreak than schools with them, revealed a separate report on Arizona's two most populous counties.

Opening of school, for the new academic year, has already shown a surge in infections among children.

More than 900,000 students in 44 states had been affected by closures between August 1 and mid-September, the CDC report revealed.

For about 17 per cent of US counties, pediatric cases rose after schools reopened. And counties without mask requirements saw larger increases - about 18 cases per 100,000 more - than those with them. Among the counties the CDC studied, a majority - about 62 per cent - did not have school mask mandates, the report said.

While "the results may not be generalisable" yet, "school mask requirements, along with other prevention strategies, including COVID-19 vaccination, are critical to reduce the spread of COVID-19 in schools," the CDC said.

Although polls show that a majority of parents support mask requirements - and despite recommendations from paediatricians and the CDC - schools remain bitterly divided over whether to implement them. Opponents of mask mandates say parents should get to decide whether their children wear them, the report said.

Various studies have shown evidence supporting the efficacy of masks in reducing the spread of the coronavirus, including with children and within schools. In July, the CDC and the American Academy of Pediatrics recommended universal masking in schools to ward off the more contagious delta variant. In North Carolina, researchers closely tracking 100 school districts with mask mandates from March to June found very little transmission in schools.

The new CDC report, despite its limitations, represents "a meaningful contribution" to the existing body of research, Danny Benjamin is a pediatrics professor at Duke University was quoted as saying.

"It's the first publication that studies the delta variant in American schools that compares schools with and without a mask policy," Benjamin said.