South Korea is urging the North to withdraw a threat that has prompted airlines to re-reroute some flights away from North Korea's airspace.
Singapore Airlines, the world's biggest by market value, said it was joining South Korean carriers Korean Air and Asiana Airlines in avoiding North Korean airspace as a precaution. Other regional carriers were not altering their flight paths.
North Korea, which is preparing to test its longest-range Taepodong-2 missile, said on Thursday it could not guarantee the safety of the South's commercial flights off the east coast of the peninsula, where the missile base is located.
It linked its warning to joint US-South Korean military exercises which start on Monday and have been held for years without major incident. The North regularly criticises them as a prelude to invasion and war.
South Korea told the North on Friday to immediately withdraw the threat it made against the South's commercial airliners.
US special envoy for North Korea Stephen Bosworth criticised Pyongyang for the aircraft threat as unacceptable provocation.
South Korea's Unification Ministry spokesman Kim Ho-nyeon said about 33 daily flights approached the South from the east with about 15 of them by South Korean airliners.
The BBC reports that passenger planes normally leave Seoul for eastern US by swinging north over the Sea of Japan to follow the Korean coastline towards Russia and North Alaska.
