The United States and Japan maintain an elaborate eavesdropping network that provided President Reagan and U.S. policy-makers a step-by-step account of Wednesday's Soviet attack on a Korean Air Lines passenger jet in the Sea of Japan, intelligence sources said yesterday.

White House spokesman Larry Speakes provided the clue that Japanese monitoring stations provided most of the information when he told reporters in Santa Barbara that the U.S. government's delay in confirming the incident was caused in part by the need to translate the intercepts from Russian to Japanese to English.

U.S. and Japanese radio-monitoring stations routinely record conversations between Soviet pilots and ground commanders in the area where the plane was downed because the Soviets have so many military facilities there.

The Kamchatka Peninsula area to the northeast is especially interesting to western and Japanese intelligence because Petropavlovsk, capital of Kamchatka, is the home of the Soviets' Pacific submarine fleet and several naval air bases.

In a report this year on Soviet military power, the Pentagon said Soviet Delta-class missile submarines patrol the Sea of Okhotsk between Kamchatka and Sakhalin Island. Soviet missile test firings around Kamchatka are another reason that electronic ears are tuned to the area.

The Japanese Defense Agency said this week that the Soviets have moved a squadron of at least 10 MiG23 fighters to Etorofu Island south of Sakhalin and about 150 miles east of Japan's northernmost island, Hokkaido.

Intelligence sources said it is likely that Japanese monitoring posts on Hokkaido provided intercepts of conversations among eight MiGs trailing the Korean Air Lines 747 and their ground commanders.

The U.S. Air Force's 6920th Electronic Security Group on Hokkaido also could have intercepted some of the messages that Secretary of State George P. Shultz relied upon yesterday in assailing the Soviets, the sources said.

Monitoring-station operators write what they call a "gist" for their superiors when they hear something considered alarming such as the conversations between the MiG pilots and their controllers. The gist usually triggers high-level attention and priority transcription by intelligence agencies of pertinent parts of the tape.

The sources said the Soviet controllers followed their practice of radioing information about intruders from the border air bases up the command chain as far as Moscow, giving U.S. and Japanese eavesdroppers several chances to record the messages.

Shultz and other administration officials stopped short of saying they had heard ground commanders give an order to fire on the 747. This and the fact that Shultz did not mention one in his unusually detailed presentation of information considered highly sensitive within the intelligence community suggests that no such order was intercepted, the sources said.

Shultz, who told a news conference that the Soviets had tracked the 747 by radar for 2 1/2 hours, could have known this because of the radio intercepts or because of radar readings by U.S. and Japanese monitoring stations on Hokkaido, the sources said.