VIENNA (Reuters) - Iran has tripled its monthly production of higher-grade enriched uranium and the U.N. nuclear watchdog has "serious concerns" about possible military dimensions to Tehran's atomic activities, the agency's chief said on Monday. As International Atomic Energy Agency head Yukiya Amano voiced his concerns at an IAEA meeting, U.S. President Barack Obama told Israeli Prime Minister Benjamin Netanyahu Washington "has Israel's back" and would not let Iran get nuclear arms. Amano told a news conference at the IAEA in Vienna there were indications of unspecified "activities" at an Iranian military site which his inspectors want to visit as...