A senior State Department official, calling the military balance on the Korean peninsula "extremely serious and potentially unsettling," has pledged that North Korea will not be permitted to gain major advantage through a new military supply arrangement with the Soviet Union.

Assistant Secretary of State Paul D. Wolfowitz, speaking to a conference on U.S.-Korea security relations here Monday evening, pledged that the United States and South Korea's combined forces will maintain "a qualitative edge" despite North Korea's acquisition of MiG23 warplanes from the Soviet Union.

U.S. military officials disclosed last month that North Korea this spring received six MiG23 warplanes, believed to the first installment of a substantial supply of more sophisticated aircraft than the North has had.

The North Koreans have now received about 18 of the advanced aircraft, according to U.S. sources, who anticipate the eventual delivery of about 40 MiG23s.

The U.S. Air Force maintains a force of 48 F16 jets at Kunsan Air Base, South Korea, and has agreed to supply 36 F16s to the South Korean Air Force beginning next April. The F16 is rated as considerably more sophisticated than the Soviet MiG23.

Wolfowitz's comments came as an unusually high-ranking Soviet delegation arrived in Pyongyang to celebrate the 40th anniversary of Korean liberation from Japanese rule at the end of World War II.

The prominence of the delegation, headed by Politburo member Gaidar Aliyev and Deputy Defense Minister Marshal Vasiliy Petrov, is the latest sign of improved relations between Moscow and Pyongyang.

The warming trend between the two communist powers, who have often had differences, seems to have begun with North Korean President Kim Il Sung's visit to Moscow in May 1984. This was Kim's first visit to the Soviet capital in nearly 20 years.

Some U.S. officials have described the warming with Moscow as a "pendulum swing" that may be partly a reaction to unhappiness in Pyongyang with foreign and domestic policies of its other major ally, the People's Republic of China.

Wolfowitz, in a reference to this communist triangle in northeast Asia, said "the competition and rivalry between China and the Soviet Union for influence in the North more often than not seems to be a prevailing if not controlling factor in their conduct and in their policy."

The senior Asian-affairs official at the State Department said the rivalry must be taken into account in assessing recent Chinese and Soviet gestures toward South Korea and "a new willingness" by China to engage in serious dialogue with the United States about the Korean peninsula and "the need to reduce tensions there."

In addition to the MiG23 supply, other signs of improved Soviet-North Korean relations include:

*A clear-cut change in North Korean official statements about the Soviet Union, described by a U.S. expert as "a deliberate effort at wooing of the Soviets" and a subsequent and lesser improvement in Soviet statements about North Korea.

*Reported Soviet military overflights of North Korean territory north of the demilitarized zone to the Yellow Sea bordering China. These unusual flights, in the opinion of some observers, may be among Moscow's immediate benefits from the improvement in relations.

*Negotiation and ratification of a treaty establishing the boundary between the two states and of consular agreement between them.

Wolfowitz said North Korea, with a 700,000-man military, has at least a 2-to-1 advantage over the 540,000-man South Korean force in tanks, long-range artillery and armored personnel carriers.

Other serious concerns, Wolfowitz said, are that North Korea has "perhaps the world's largest commando force, designed for insertion behind the lines in time of war" and that it has been moving more troops closer to the Demilitarized Zone that separates it from the South.

The U.S. official gave a guarded assessment of the political and economic dialogue between North and South Korea, which began anew last fall and is being continued by several Korean committees.

"Little that has come of the talks so far is inconsistent with the most skeptical and fundamental interpretation of North Korean motives . . . an effort to encourage a premature and unwarranted relaxation and to encourage divisions between the United States and our allies in South Korea," Wolfowitz said.

Two other possible North Korean motives for renewal of the talks, according to Wolfowitz, are "severe economic stagnation" forcing a search for new approaches and the diplomatic isolation imposed on the North following the 1983 terrorist bombing in Rangoon that killed 17 South Korean officials.

The United States supports the North-South talks and is "encouraged" that the direct discussions are taking place, he said.