July 14 (Bloomberg) -- Two South Korean fighter planes crashed into the sea in separate incidents yesterday, the Ministry of Defense said. The four pilots are missing and the causes of the crashes are under investigation, the ministry said in a statement.

An F-4E jet fighter disappeared from radar about 8:40 p.m. yesterday while flying over the southern waters of the Korean peninsula, the ministry said. Part of the jet's wreckage was found, it said. An F-5F jet fighter went down in the western Yellow Sea at 8:48 p.m., according to the statement.

The F-4E jet fighters were first produced during the 1960s in the U.S. and were deployed in South Korea during the 1970s, the ministry said. The South Korean Air Force has about 80 of these jets. The F-5F was deployed in South Korea during the 1980s.

South Korea is planning to receive 40 F-15K fighters it ordered from Boeing Co. in 2002 starting this year through 2008. The government said last year it wants to buy 40 additional F-15s to reinforce the country's defense and replace aging jets.

South Korea needs the jets to counter a long-range artillery threat from North Korea. The government bought fewer jet fighters in 2002 than the originally planned 120 because of the 1997-1998 Asian financial crisis.

To contact the reporter for this story: Heejin Koo in Seoul at hjkoo@bloomberg.net

Last Updated: July 13, 2005 21:48 EDT