Moscow on Tuesday angrily ordered foreign military attaches in the country to confirm or refute "outrageous" reports that Russian air strikes have caused massive civilian deaths in Syria.

Deputy defense minister Anatoly Antonov contacted the military attaches of several Western countries as well as Turkey, Saudi Arabia and NATO over international media coverage of the four-week-old bombing campaign, the defense ministry said.

At the center of Antonov's "outrage" are reports such as those by the Syrian Observatory for Human Rights, which revealed last Friday that over a third of the casualties in Russian strikes are civilians.

"We are accused not only of hitting the 'moderate' opposition but also civilian targets such as hospitals and also mosques and schools," a ministry statement quoted Antonov as saying. "As a result of this, according to Western media reports, non-combatants are allegedly being killed."

The reports were derided as "anti-Russian smears" by Antonov, who added, "today we invited the US, British, French, German, Italian, Saudi, Turkish and NATO military attaches to provide an official explanation of the substance of the statements made, or to refute them."

"This particularly concerns the outrageous accusations in a number of English-language media outlets of alleged strikes on hospitals," Antonov said.

As he noted, the Syrian-American Medical Society last Thursday revealed that the Russians have already bombed nine hospitals or field clinics.

If Russia "is not presented with evidence or official refutations, we will consider that these anti-Russian smears are part of an information war against Russia," Antonov said.

He added that he expected to hear back from the attaches in "a few days."

Russian air strikes in Syria have killed at least 446 people, more than a third of them civilians, the Syrian Observatory for Human Rights monitoring group said last Friday.

Last Tuesday the group revealed that of the 370 people killed up to that point, only 52 were from Islamic State (ISIS), while 127 were civilians, including 36 children and 34 women, and the rest are largely Western backed rebel forces.

Russia began its air campaign in Syria on September 30 in support of its ally President Bashar al-Assad.

The campaign has been criticized not only for causing civilian casualties but also for targeting non-jihadist rebel groups more often than the Islamic State group or Al-Qaeda.

AFP contributed to this report.