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A senior defense official has told NBC News that the Russian military has been moving artillery and ground forces towards Hama — one of the locations that the Russians targeted in the first 24 hours of their airstrikes.

"So much for fighting ISIS," the defense official said.

Related: Syria War: Russia Insists Its Warplanes Are Bombing ISIS Targets

U.S. officials have questioned whether Moscow’s airstrikes — in the name of defeating ISIS — were genuinely targeting the group or were instead a cover to shore up Syrian President Bashar Assad by bombing anti-government rebel groups. Russia defended its latest round of airstrikes in Syria amid mounting questions over their targets, insisting that Moscow's planes had struck ISIS installations.

The U.S. official also said that after Russian aircraft breached Turkish airspace by roughly 20 miles, Russian Foreign Minister Lavrov contacted the Turkish government to apologize and acknowledge that it was a mistake.

Related: DoD Sec. Carter: Russia's Syrian Approach 'Doomed to Fail'