The Russian military suffered a setback in Syria when fires destroyed four Mi-24 attack helicopters last week, but U.S. officials tell Fox News the intelligence community believes it was an "accident," not an Islamic State terror attack.

The fires also destroyed more than a dozen trucks at an air base in Tiyas, roughly halfway between Homs and Palmyra.

On May 14, ISIS claimed responsibility for the attack on the Russian base, known locally as 'T4.' Fighter jets belonging to the Syrian regime were also destroyed.

On Tuesday, the U.S. intelligence firm Stratfor published satellite images showing the destruction to part of the base including the charred remains of the Russian attack helicopters and a supply depot. Stratfor analysts said they believed the ISIS claims were valid.

U.S. officials told Fox News that intel teams more strongly believed an accidental fuel tank explosion caused the damage. It was unclear what could have triggered the blast, which set off secondary explosions on the runway.

"The Stratfor analysis is wrong," one U.S. official told Fox News. Another official said, "There is no indication that an ISIS attack took place."

When asked about the cause, another official said, "Refueling can be dangerous."

A spokesman for the Russian Defense Ministry Tuesday said all of Russia's helicopters were accounted for and the images of the scorched runway wereover a month old, the product of previous attacks by the Syrian regime and "terrorist groupings."

Major-General Igor Konashenkov added, "The rumors about the destruction of a group of Russian helicopters and two dozen trucks were created by the propagandists of [ISIS] who unsuccessfully tried to 'sell' this alleged piece of news about ten days ago," according to TASS, a Russian news agency.

Bombings claimed by ISIS in Latakia and Tartus, Assad regime strongholds along the Mediterranean coast, killed more than 190 Syrians this week.