Human rights watchdog Amnesty on Wednesday accused Russia of using cluster munitions and unguided bombs on civilian areas in Syria in attacks that it says have killed hundreds of people in the past few months, which may amount to war crimes.

The report by Amnesty International said there has been a surge in reports of cluster munitions dropped in areas targeted by Russian forces since Moscow formally joined the conflict Sept. 30.

Cluster munitions are by nature indiscriminate and often leave unexploded bomblets on the ground. These can maim and kill civilians long after the cessation of hostilities.

The report focuses on six attacks in Homs, Idlib and Aleppo provinces between September and November which it says killed at least 200 civilians. It denounced Russia's "shameful failure" to acknowledge civilian killings.

"Some Russian air strikes appear to have directly attacked civilians or civilian objects by striking residential areas with no evident military target and even medical facilities, resulting in deaths and injuries to civilians," said Philip Luther, director of Amnesty's Middle East and North Africa program. "Such attacks may amount to war crimes," he said.

The accusations follow a report by New York-based Human Rights Watch last week which said cluster munitions were used on at least 20 occasions since Syria and Russia began their joint offensive on Sept. 30.

Russian officials have repeatedly rejected such accusations, and residents and opposition activists inside Syria acknowledge they have no way of knowing for sure whether the attacks are carried out by Russia or the Syrian military.

Russia began air operations in Syria on Sept. 30 after receiving parliamentary approval. The Kremlin claimed the airstrikes, which followed a military buildup in Syria, aimed to support the forces of Syrian President Bashar Assad, a long-standing Russian ally, against Daesh. However, Turkey and the West have accused Russia of targeting moderate fighters opposed to Assad, many of which are supported by Turkey and the U.S.