Iran has installed chemical weapons and 6,000 troops at the mouth of the Persian Gulf, Defense Secretary William Perry told reporters today. On a six-day tour of the region, Perry warned that the military buildup could be used to block the Gulf shipping lanes that are used to transport half the world's oil. He is urging Arab allies to upgrade their defenses and improve ties with the U.S. military in the face of growing military arsenals in bothIran and Iraq. Perry has warned repeatedly in recent days ofIran's military stockpile, including anti-ship and anti-aircraft missiles, near the mouth of the Gulf. He had not previously mentioned chemical weapons.TIME Defense correspondent Mark Thompsonsays Perry is determined to show Iran and Iraq that the U.S. will not desert its Gulf allies if they come under attack.