Air raid sirens wailed across Seoul this afternoon as a Chinese MiG19 fighter-bomber flew into South Korea. The government said a Chinese pilot had defected.

South Korean jets escorted the plane to an air base after it wagged its wings to indicate intention to defect, according to a Defense Ministry statement. It was the fifth defection here by Chinese military personnel in less than four years.

Starting at 2:30 p.m., radios and street loudspeakers ordered the city's nearly 10 million citizens to remain orderly and prepare to evacuate. The warnings came minutes after radar spotted the jet approaching South Korea's west coast.

Some people responded according to instruction. In Seoul's express bus terminal, for instance, about 1,000 persons reportedly were evacuated into an underground shopping arcade.

But on the streets and in offices, others simply milled about, waiting for further news. When the alert was called off, the government gave prompt word by radio that it had been caused by the Chinese aircraft.

The incident came only several days after a routine air raid drill and left many people breathless. Communist North Korea is only a few minutes' flying time from Seoul and the two intensely hostile governments are on a permanent war footing.

At the same time, South Korean radar detected two jets headed from the Haeju area of North Korea in the direction of the south, the Defense Ministry said. South Korean jets scrambled, and the two aircraft in the north turned back, according to the ministry.

The South Korean Air Force tonight identified the Chinese pilot as Chen Bao Chung. It said he had been on a training mission from a base at Shenyang in Manchuria.

In 1982, a Chinese pilot flew his MiG19 here, followed in 1983 by another pilot with a MiG21. In March last year, a Chinese torpedo boat arrived with 13 crewmen and six dead following a mutiny.

Six months ago, a Chinese light bomber with a crew of three crash-landed in a South Korean rice field. In addition, in 1983 six Chinese hijackers brought a Chinese civilian airliner to South Korea with more than 100 aboard.

In recent years, South Korea has been trying to forge contacts with China, which fought on the side of the North during the Korean war and is still technically in a state of war with the South. The defections appear to have both helped and hindered that effort.

The South has angered China by generally allowing those who so request to go on to Taiwan, which pays large rewards to defecting pilots who bring Chinese warplanes.

The torpedo boat case was the one exception. South Korean authorities announced that the mutiny was not politically motivated and sent the two mutineers back to China along with other crew members.

At the same time, negotiations over the return of people and equipment have forced face-to-face contacts. After the hijacking, a Chinese delegation, the first to visit South Korea, negotiated the return of the plane, passengers and crew