IRAN will respond promptly to any threat against its security, a foreign ministry spokesman has said, after the United States, Israel and Britain blamed Tehran for an attack on an Israeli-managed tanker off the coast of Oman. Tehran has denied any involvement in the attack on Thursday in which two crew members - a Briton and a Romanian - were killed. The United States and Britain said on Sunday they would work with their allies to respond to the attack on Mercer Street, a Liberian-flagged, Japanese-owned petroleum product tanker managed by Israeli-owned Zodiac Maritime. Britain summoned the Iranian ambassador on…