BEIRUT (Reuters) - The Syrian army and its allies have captured areas of Islamic State-held al-Mayadin in eastern Syria, a Hezbollah-run military news outlet reported on Thursday, in an advance into the jihadist group s main urban base of operations in Syria.  Backed by Russia, Iran and Shi ite militias, the Syrian army encircled the militants in al-Mayadin on Sunday. In June, two U.S. intelligence officials told Reuters that they believed Islamic State had moved most of its diminished command structure and propaganda team to al-Mayadin, southeast of its former capital of Raqqa. 