BEIRUT - US-backed fighters besieging Islamic State's final shred of territory in eastern Syria said they had pushed further into the enclave following clashes and air strikes by the US-led coalition."Our fighters advanced deep into areas controlled by the terrorist organization and established a number of new points, following clashes in which terrorists sustained a number of deaths and injuries," a statement from the Syrian Democratic Forces (SDF) press office said.It said 15 members of Islamic State had been killed after they tried to attack SDF troops