The chief of Japan's defense forces today expressed concern that the Soviet Union may have established a military air base in Vietnam.

Ganri Yamashita, director general of Japan's Defense Agency, said the presence of a new base is still conjectural but said Japan "cannot ignore the possibility." If the suspicion is true, he added, it would be of "great concern" to Japan.

Yamashita said a base in Vietnam would affect the security of all Asia and would have a "severe impact" on the sea lanes in Southeast Asia through which Middle Eastern oil reaches Japan. Japan imports virtually all of its oil.

The civilian defense chief said in an interview that he based his suspicion on the pattern of recent flights past Japan by Soviet long-range reconnaisance planes.

Military intelligence sources reported that on Wednesday two Soviet Tupolve 95D reconnaissance planes, called "Bears," flew south over the Tsuhima Strait that separates Japan from the Korean peninsula.

They were tracked for three hours by Japanese radar before disappearing in the direction of Southeast Asia and have not been sighted returning to their Soviet bases.

During the recent war between Vietnam and China, Japanese intelligence reported at least seven such flights by similar planes. It was assumed then that they were intended as a show of support for the Soviet Union's ally, Vietnam, and as a warning to China.

In all of those cases, the aircraft returned to their bases on the same day. Japanese radar tracked them going south and returning home.

Yamashita said that although there are "other possibilities," he assumes now that the last two planes landed in Vietnam. If so, he said, it would be the first time that the long-range Bears, loaded with electronic reconnaissance gear, have been based there.

During the China-Vietnam border war that began in February a number of Soviet transport planes carrying equipment and supplies landed at Vietnamese airfirelds.

"This is totally different from the transport planes," Yamashita said, referring to the recent flight. "These are very long-range electronic reconnaissance planes."

Emphasizing that he was speculating without solid evidence, the director general linked the possible air base with the recent arrival in the Indian Ocean of the new Soviet aircraft carrier Minsk and said it is "quite possible) that the Minsk is headed for the Pacific. He said there may be a connection between the location of the Bear reconnaissance planes and the Minsk.

Japan's entire defense strategy is based on the assumption that the Soviet Union is the major potential enemy and all Soviet military movements in Asia arouse intense concern, no matter how remote.

Until the last suspicions about a base in Vietnam, Japan had been made nervous by Soviet reinforcement of two islands lying just a few miles off Japan's northern main island of Hokkaido. Both the Soviet Union and Japan claim the islands but the Soviets have occupied them since World War II.

The Soviets have replied to past Japanese alarms with charges that they reflect a revival of "militarism" in this country. Some Western sources believe the strengthened Soviet forces are intended not as a threat to Japan but as part of a process of encircling China.

Western sources here said they know of no evidence that Soviets are building an air base in Vietnam.

Yamashita said he does not believe the Soviet military intentions are limited merely to the troubles between China and Vietnam. He pointed to the incident of the reconnaissance planes, the nearness of the carrier Minsk, and the possibility, reported in Washington, that the Soviets may deploy their new Backfire bombers in the Far East.