Greek air traffic controllers went on strike on Thursday over communications systems they say are so bad that songs from radio stations often interrupt conversations with pilots.

Air traffic controllers have staged repeated strikes since the start of the year over the issue, and over radar systems they say sometimes fail.

"We sometimes hear hymns or traditional songs in the background," said Panagiotis Hatzakis, a member of the board of the air traffic controllers' union.

"This is dangerous, especially in the summer months, when air traffic is heavier."

Between 2,300 and 2,900 planes on average fly through Greek airspace every day, the union said.

All but emergency flights were suspended between 0500 and 0900 GMT on Thursday. Aegean Airlines cancelled 21 domestic flights, while Olympic Air cancelled 32 flights, including scheduled flights from international destinations such as Germany, Italy and France.

The head of the air traffic controllers' union said the main radar systems used to track aircraft also sometimes fail.

"(The system) goes dead... we have a back-up system of course, but it is not supposed to be relied on for hours," Angelos Sotiropoulos, head of the union, told a news conference.

Air traffic controllers vowed to continue with protests unless the government takes action.

Greece's transport ministry declined to comment.