The United Arab Emirates said on Sunday its air defence systems were responding to incoming missiles and drones, while explosions were heard in Qatar's capital Doha and the government sent security alerts to residents' mobile phones, according to Reuters witnesses, as tensions in the Gulf escalated following fresh U.S. strikes on Iranian targets.The UAE Defence Ministry said its air defence systems were engaging missiles and drones, while the National Emergency Crisis and Disaster Management Authority urged residents to remain calm and follow official instructions.The ministry later said the country's air defence systems were continuing to respond to a missile threat.Meanwhile, explosions were heard in Doha and the Qatari government sent security alerts to residents' mobile phones, according to Reuters witnesses, heightening concerns that the conflict was spreading across the Gulf.The developments came hours after the United States launched a fresh round of strikes on Iranian targets, saying Tehran had "blatantly attacked" a Cyprus-flagged container ship transiting the Strait of Hormuz.Iran had declared the Strait of Hormuz closed "until further notice", saying a vessel had travelled on an unauthorised route and warning that any retaliation over the incident would be met with a "severe response."The latest escalation has raised concerns over the security of Gulf states and the safety of global energy supplies, with the Strait of Hormuz serving as a key route for global oil and gas shipments.(With inputs from Agencies)