Moscow is sending an advanced anti-aircraft missile system to Syria, two Western officials and a Russian source said, as part of what the West believes is stepped-up military support for embattled President Bashar Assad.The Western officials said the SA-22 system would be operated by Russian troops, rather than Syrians. It 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.A US official separately confirmed the information.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.