WASHINGTON--Photographic evidence shows Libya lied when it claimed the two planes that the United States shot down over the Mediterranean Sea were unarmed reconnaissance aircraft, the Reagan Administration said yesterday.

Citing video footage from one of two U.S. Navy F-14 jet fighters involved in the confrontation Wednesday, Pentagon spokesman Dan Howard said one of the Libyan fighters carried four missiles on its fuselage and wings.

"They've gone out yesterday and said repeatedly that those were unarmed reconnaissance aircraft. Well, we have the pictures now that they were not unarmed aircraft," Howard told a Pentagon briefing.

"They were obviously armed aircraft with obviously hostile intent. We believe that our aircraft commanders behaved in a prudent manner in defending themselves, in defending their aircraft and in defending their ship," he said.

Pentagon officials, after unveiling the cockpit tape, announced they were also releasing a still picture taken from the video tape to prove the Libyan MiG-23 Flogger jets were armed.

The tape supports additional assertions of administration officials that the two F-14's repeatedly tried to maneuver away as the Libyan fighters pursued them; the pilot of the lead F-14 can be heard counting each time he tries to head away, only to have the "bogies jink back into us."

Finally, the lead pilot gives the order to fire missiles.

Although the quality is not good, the tape concludes with a dramatic rush of pictures at the end of the dogfight--when the last Libyan MiG is suddenly within visual range.

The audio portion of the cockpit tape is about 7-1/2 minutes long, while the video portion lasts less than 30 seconds. The audio portion shows that early on in the confrontation, the American aviators became concerned about the Libyans' intent.

At the United Nations, the Soviet ambassador, Alexander M. Belonogov, told reporters Thursday that Moscow "gave a warning signal to Washington," before the American jet fighters shot down the Libyan planes, to show restraint and "not to raise the tension in the area." He refused to elaborate.

"We deplore this accident," Belonogov said. "It shows the seriousness of the situation and that all military personnel should be under strong instructions not to be too quick to pull the trigger."

The United States told the U.N. Security Council that the U.S. planes fired on the Libyan aircraft in reaction to hostile actions that "constituted an armed attack" on American forces.

But Libyan Ambassador Ali Sunni Muntasser called the incident a "premeditated act of aggression" and said the Libyan fighters were "unarmed, on routine reconnaissance, ordinary flying."

President Reagan, boarding his plane in Los Angeles to return to Washington following a vacation, told reporters, "Our pilots acted completely in self-defense."