In the first quarter of 2016, 26 percent of Russian airstrikes in Syria targeted IS, according to the IHS Conflict Monitor.
That dipped to 22 percent in the second quarter, and 17 percent in the third quarter.
"Last September, President (Vladimir) Putin said it was Russia's mission to fight international terrorism and specifically the Islamic State," said Alex Kokcharov, principal Russia analyst at IHS Country Risk.
"Our data suggests that is not the case.
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"Russia's priority is to provide military support to the Assad government and, most likely, transform the Syrian civil war from a multi-party conflict into a binary one between the Syrian government and jihadist groups like the Islamic State."
This had the effect of "undermining the case for providing international support to the opposition", he said.
Russia on Saturday vetoed a UN draft resolution demanding an end to the bombing of Aleppo, which has escalated since the Russian-backed Syrian army launched an offensive against rebels in the city last month.
IHS also confirmed that territory controlled by the IS group in Iraq and Syria continues to shrink, from about 68,300 square kilometres (26,370 square miles) in July to about 65,500 square kilometres last week.