Jan 4 (Reuters) - Flights across Greece were grounded on Sunday after a collapse of radio frequencies crippled air traffic communication, stranding thousands of travellers and bringing airport operations to a halt.

There was little clarity on what caused the disruption, which began early Sunday and quickly escalated. Some overflights across Greek and regional airspace were still being serviced, but restrictions were imposed on airport operations for safety reasons, Greece's civil aviation........

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