Russia Denies That It Launched Airstrikes in the Ancient Syrian City of Palmyra

Russia has denied that its warplanes launched airstrikes against the so-called Islamic State (IS) in the Syrian city of Palmyra on Tuesday, contradicting Syrian news reports and the Syrian Observatory for Human Rights, a UK-based monitoring group.

The strikes, which also hit targets in Homs and Raqqa, reportedly hit three weapons depots and 12 vehicles, killing at least 19 IS fighters. Fifteen of the fighters were killed in Palmyra, according to the Observatory.

Russia began its aerial campaign in Syria last week with the stated intention of weakening IS and breaking the group's grip on parts of the country, but many of the strikes have reportedly targeted other rebel groups opposed to Syrian President Bashar al-Assad.

Related: Why the Hell Did Russia Intervene in Syria?

Russian officials denied that the country was responsible for the attack on Palmyra, saying the city is too residential and culturally significant for strikes.

"All reports by foreign media that Russian planes have allegedly conducted airstrikes against the city of Palmyra are absolute lies," Russian spokesman Igor Konashenkov said, according to Syrian news station Al-Manar. "Our aviation in Syria does not attack residential areas nor — especially not — the architectural monuments there."

Rami Abdulrahman, the director of the Syrian Observatory for Human Rights, told Reuters the Russian strikes were the heaviest yet against Palmyra, the ancient desert oasis captured by IS in May.

Since seizing control, IS has destroyed many of Palmyra's architectural and cultural antiquities, including blowing up the city's Arch of Triumph, a Roman monument, earlier this week. Militants also blew up two major temples in the UNESCO World Heritage site, the Temple Bel and the Baal Shamin Temple, claiming the landmarks were idolatrous.

The reports of airstrikes in Syria come on the heels of news that Russian planes also violated Turkey's airspace over the weekend, an incident that Moscow claimed was an accident.

Related: Russia's Violation of Turkish Airspace 'Does Not Look Like an Accident'

NATO Secretary General Jens Stoltenberg said there has been a "substantial buildup" of Russian air defenses, ground troops, and naval presence around Syria in recent days. Russia's Defense Ministry said on Sunday that its forces were able to strike 10 IS targets over the weekend.

"We have managed to disrupt their control system, the terrorist organization's supply lines, and also caused significant damage to the infrastructure used to prepare acts of terror," the Russians said in a statement

Assad has expressed support for the airstrikes, calling the Kremlin's move to get involved in the conflict vital to saving the Middle East from destruction.

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Reuters contributed to this report.