More than a third of Saudi-led air strikes on Yemen have struck civilian sites including schools, hospitals and mosques, according to a survey of the conflict published in the Guardian on Friday.

The findings came from the Yemen Data Project, a group of security and human rights experts, who examined more than 8,600 air raids in the campaign between March 2015 and the end of August this year.

Out of these it found 3,577 were listed as hitting military sites and 3,158 non-military, while 1,882 strikes were classified as unknown, the Guardian said.

Over the course of the campaign led by Saudi Arabia, the survey listed 942 air raids on residential areas, 114 on markets, 34 on mosques, 147 on school buildings, 26 on universities and 378 on transport.

The study, which the report said was based on open-source data including research on the ground, showed that one particular school building was hit nine times, while one market was hit 24 times.

The project said the coalition hit more non-military sites than military in five of the last 18 months.

Saudi Foreign Minister Adel al-Jubeir was quoted by the Guardian as dismissing the report as "vastly exaggerated" and challenging its methodology.

He said rebel rebel fighters had "turned schools and hospitals and mosques into command and control centres. They have turned them into weapons depots in a way that they are no longer civilian targets.

"They are military targets. They might have been a school a year ago. But they were not a school when they were bombed," he said.

Saudi Arabia intervened in Yemen in March 2015 to prop up the government of President Abedrabbo Mansour Hadi after Shiite Huthi rebels took over the capital Sanaa.

Since then the conflict has left more than 6,600 people dead, most of them civilians, and displaced at least three million others, according to the United Nations.

A UN report in June found the coalition responsible for 60 percent of the 785 deaths of children in Yemen last year.

Fighting has intensified since the collapse of UN-backed peace talks in Kuwait on August 6.