* U.S. forces hit fighters in **rth Iraq and south of Baghdad * Iraqi Kurds say they have retaken half the ground lost in early August * Fighters keep up siege on Syrian Kurdish city near Turkish border By Isabel Coles and Jonny Hogg ARBIL, Iraq/MURSITPINAR, Turkey, Sept 30 (Reuters) - Iraqi Kurdish forces captured a strategic border crossing and several villages from Islamic State fighters in **rthern Iraq on Tuesday, scoring gains as the militants were pounded by heavy U.S.-led air strikes and the Iraqi army advanced from the south. ...