Thousands of travellers in Greece faced huge delays after flights stopped because of a sudden, unexplained loss of radio frequencies around 0700 GMT on Sunday. "No plane landed or took off for at least two hours," said the Athens airport press office, as quoted by AFP. The cause of the disruption remains unclear. Greece’s civil aviation authority said while some flights flying over Greece were allowed, airport operations were restricted for safety. More than 75 flights were delayed. Panagiotis Psarros, chair of the Association of Greek Air Traffic Controllers, told state broadcaster ERT, "For some reason all frequencies were suddenly lost … We could not communicate with aircraft in the sky." He called the incident "very serious" and blamed "obsolete" airport equipment. The problem seemed to start with a collapse of central radio frequency systems at Athens and Macedonia control centres. The Athens centre controls a large part of Greek airspace. Psarros added, "We haven’t been informed about the cause of this problem… certainly the equipment we have is virtually ancient. We have raised this many times in the past." A Transport Ministry official told Reuters some aircraft heading north and east were allowed to leave, but flights were delayed overall. In Israel, the Airports Authority said Greek airspace was closed until 4 p.m. local time (1400 GMT), warning of arrival and departure delays.