BEIRUT/JERUSALEM, Sept 24 (Reuters) - An Israeli airstrike on Beirut killed a senior Hezbollah commander on Tuesday as cross-border rocket attacks by both sides increased fears of a full-fledged war in the Middle East and Lebanon said only Washington could help end the fighting. Hezbollah early on Wednesday confirmed senior commander Ibrahim Qubaisi was killed by Israeli airstrikes on Tuesday on the Lebanese capital as Israel announced earlier. Israel said Qubaisi headed the group's missile...