The UN body for aviation safety has urged North Korea to retract a threat to South Korean passenger aircraft, Seoul's foreign ministry said Tuesday.

The International Civil Aviation Organisation (ICAO) "unanimously" decided Monday to send North Korea a letter calling on it to withdraw its March 5 announcement, the ministry said in a statement.

The North last Thursday announced it "can no more assure the safety of South Korean passenger flights" over its east coast, in protest at a joint US-South Korean military exercise that started this week.

Flag carriers Korean Air and Asiana immediately began re-routing flights away from North Korean-controlled airspace, leading to longer journey times and higher fuel bills.

Some 200 flights will be diverted during the March 9-20 exercise.

The ICAO letter calls the North's announcement a "grave threat" to the safety and security of international civil aviation and urges it to abide by global aviation rules, the ministry said.

The communist state's announcement was the latest in a series of threats that have raised tensions in recent weeks.

The ministry said 28 of the 32 members of the ICAO council, including China and Russia, made comments in support of the letter at a council meeting.

The headquarters of the Montreal-based body could not immediately be reached for comment. Mokhtar Awan, Bangkok-based regional director of the ICAO, told AFP he had no information on the matter.