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 authority said. Dozens of flights were disrupted.

"For some reason all frequencies were suddenly lost... We could not communicate with aircraft in the sky," Panagiotis Psarros, chair of the Association of Greek Air Traffic Controllers, said.

He said the problem seemed to be a collapse of central radio frequency systems at the Athens and Macedonia area control systems, the largest air control facility in the country based in Athens.

It monitors the Athens Flight Information Region, a vast expanse of airspace under the control of Greek authorities.

Flight trackers showed Greek airspace was largely empty.

"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," Psarros said.

A transport official said some aircraft heading north and east were allowed to leave, adding that more than 75 flights were delayed. (Reuters)