According to a report by Reuters on Friday, the Western officials said the SA-22 system would be operated by Russian troops, rather than Syrians. The system was on its way to Syria but had not yet arrived.
"This system is the advanced version used by Russia and it's meant to be operated by Russians in Syria," said one of the sources, a Western diplomat who is regularly briefed on US, Israeli and other intelligence assessments.
Two US officials separately confirmed the information.
The second US official said the United States had indications that, although the entire system had not arrived, some control system components for the SA-22 had been positioned at an airfield near Latakia.
The Russian source, who is close to the Russian navy, said the delivery would not be the first time Moscow had sent the SA-22 system, known as Pantsir-S1 in Russian, to Syria. It had been sent in 2013, the source said.
"There are plans now to send a new set," the source said, without detailing how far along the process was.
However, the Western diplomat said the version of the SA-22 on its way to Syria was newer than previous missile systems deployed there.
Speaking at a news conference in Moscow, Russian Foreign Minister Sergei Lavrov said Russia was sending military equipment to Syria to help the Assad government combat ISIL terrorists, and had sent experts to help train the Syrian army to use it.