Fighter jets given to Syria by Russia in a bid to support Syrian President Bashar Assad's regime have so many mechanical issues they often are unable to hit targets, a senior Defense official tells USA Today.

Despite those issues, Russian jets are reported to have hit 94 "terrorist" targets in Syria over the past 24 hours, the highest one-day tally since Moscow launched its bombing campaign on Sept. 30.

But USA today said nearly a third of Russian attack planes and about half of its transport planes "are grounded at any time as the harsh, desert conditions take a toll on equipment and crews."

The newspaper said the Russians "appear to be having difficulty adapting to the dusty conditions, and the number of airstrikes they have conducted seems to have dipped slightly."

Russia has been making the aggressive airstrikes in support of Assad in defiance of Washington, which wants the Assad regime toppled. But the U.S. has remained on the sidelines as Russian President Vladimir Putin insists the Assad regime is fighting the Islamic State.

Meanwhile, an alliance of Free Syrian Army-related insurgent groups said on Monday it was skeptical about a Russian proposal to help rebels, and that Moscow must stop bombing rebels and civilians and withdraw its support for Assad.

Foreign Minister Sergei Lavrov said on Saturday the Russian air force is would be ready to help the "patriotic" Syrian opposition.

"Their words are not like their actions. How can we talk to them while they are hitting us?" Issam al-Rayyes, spokesman for the Southern Front of the Free Syrian Army, told Reuters.

Russian warplanes have bombed a number of FSA-affiliated groups in northern areas of Syria since intervening in the war on the side of Assad.

Reuters contributed to this report.

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