Two Korean-owned stores in upper Northwest Washington were firebombed early yesterday, bringing to 11 the number of mysterious, early morning bombings of Korean businesses in the city in 18 months, D.C. police reported.

No one was injured in yesterday's bombings, which occurred almost simultaneously about 3:20 a.m. at 502 and 443 Kennedy St. NW, less than five blocks from three other Korean businesses that have been firebombed.

As in the nine previous attacks, flammable liquids were placed against an outside door of the establishments and set afire, according to Lt. Dennis Hyater of the D.C. police arson squad. Sources said the liquid was placed in a container and some type of fuse, such as a cigarette, was apparently used to enable the arsonist to flee before the blasts.

Sung Soo Kim, owner of the Twin's Deli Market at 502 Kennedy St., said yesterday that he and his family, who live in the back of the store, were awakened by a loud explosion at the front of the building and then heard another explosion about a minute later from across the street.

Kim said he doused the blaze at his front door with a fire extinguisher while firefighters, who arrived a short time later, put out the fire across the street at Sea World (I), a new fish and vegetable market at 443 Kennedy St. that is under renovation and not expected to open for another month.

Hyater said damage to each business was less than $1,000. He said that a woman in a white dress, possibly a uniform, was seen running north on Fifth Street shortly after the blasts. He said the woman is not a suspect in the blazes, but police are searching for her as a possible witness.

The series of firebombings, the first of which occurred Dec. 12, 1984, at Rocket Liquors, 900 Kennedy St. NW, are being investigated by a task force from the D.C. police arson squad, 4th District police and the Treasury Department's Bureau of Alcohol, Tobacco and Firearms. The task force, formed after the May 30, 1985, firebombing of the Food Barn, 6203 Georgia Ave. NW, has interviewed more than 2,500 persons.

Owners or managers of all the bombed stores have been interviewed by The Washington Post. Many have suggested that the incidents are the result of tension with blacks, who may be upset at the number of Korean-owned businesses that have moved into the city in recent years.

However, investigators and other D.C. police officials familiar with the investigation have said in background interviews that they believe the firebombed businesses may be the victims of extortion by other Koreans. While stressing that they have no firm evidence, sources have said that the KK (for Korean Killers), a powerful Korean gang in Los Angeles, and the Korean Power gang in New York have used similar tactics to extort money from Korean businesses in those cities.

They said that the firebombing of the Chinese-owned General Commodities Co., 125 Missouri Ave. NW, last July 4, occurred shortly after investigators started to inquire about possible extortion in the Korean community and may have been a copycat incident or an attempt to lead police investigators astray.

Leaders of the local Korean community have hotly disputed the police theories, and Police Chief Maurice T. Turner Jr. has said that police have "no information that would substantiate the incidents are a result of organized crime or gang acitivity involving the Korean-American community."