A U.S. Army liaison officer was shot to death by a Soviet guard on what was described as a routine surveillance mission in Soviet-dominated East Germany Sunday afternoon, and the United States accused the Soviet Union yesterday of "murder."

The killing of Maj. Arthur D. Nicholson Jr. generated an exchange of charges between the two superpowers, including statements from President Reagan and State Department officials. The Soviet Embassy, while expressing "regret," released its own widely differing version of what happened.

U.S. officials said they did not believe the shooting presaged an effort by the Soviets to abandon the ground rules under which the two sides have observed one another's activities in Germany since 1947.

Reagan, while calling the killing an "unwarranted tragedy," said it makes him "more anxious to go" to a summit meeting with the new Soviet leader, Mikhail Gorbachev.

The working assumption among State Department officials was that a trigger-happy Soviet soldier was responsible. Presidential national security affairs adviser Robert C. McFarlane said "error or mistaken identity" are among the possible causes of the "unjustified" shooting being investigated.

The White House said Reagan was informed of the killing between 6 a.m. and 7 a.m. yesterday, nearly 20 hours after it took place and at least 13 hours after it was confirmed by U.S. officials who went to the scene. Spokesman Larry Speakes, in explaining the delay, said "the incident went on for a period of time."

Nicholson was part of a two-man patrol on liaison-and-surveillance duty under a 1947 U.S.-Soviet agreement providing for an exchange of such missions in East and West Germany, the State Department said. The shooting of Nicholson took place near Ludwigslust, East Germany, about 100 miles northwest of Berlin.

The U.S. account, made public here by Assistant Secretary of State Richard R. Burt, said Nicholson was 300 to 500 yards outside a "permanently restricted area" that is off-limits to U.S. liaison teams when he was inexplicably shot, without warning, by a Soviet soldier.

Burt said three shots were fired at Nicholson and his driver, Sgt. Jessie Schatz, both of whom were described as in camouflage field uniform and unarmed. Nicholson was hit in the chest, Burt said.

Schatz was not hit but was forced into his car at gunpoint by Soviet troops and prevented from administering first aid, Burt added.

The State Department official said a Soviet soldier arrived with a medical kit about 30 minutes after the shooting, but made "no effort" to give aid to Nicholson for another 30 minutes, when it was determined that the U.S. officer was dead.

Burt could give no details about when Nicholson died but said he cried out to his driver, "Jess, I'm shot," after being hit.

"There was completely no justification for the murder of Maj. Nicholson or for the use of force of any kind," said Burt.

He described as "wrong, dead wrong" a Soviet Embassy account that Nicholson and Schatz "entered the territory" of a restricted Soviet military installation "despite the presence of clearly visible warning signs in Russian and German."

Soviet Embassy spokesman Vladimir Kulagin told reporters that Nicholson was "caught red-handed by a Soviet sentry" guarding combat equipment that the U.S. officer was photographing. Kulagin said Nicholson was killed "after a warning shot while attempting to escape."

Oleg M. Sokolov, the embassy's second-ranking official, gave essentially the same version when he went to the State Department at his own request early yesterday to discuss the killing, U.S. sources said.

Burt said the Soviets "expressed regret" about Nicholson's death, a fact confirmed by Kulagin.

"Like the Korean Air Line shootdown, they now shoot first and ask questions later," said a State Department official of the Soviet action.

Unlike its stance after the KAL shooting, the administration seemed to take pains to limit the vehemence of its public comment, despite the provocative nature of the Soviet Embassy statement.

"We view this as a serious situation," said Burt in a mid-afternoon news conference. He declined to draw conclusions about the potential impact on U.S.-Soviet relations in a period that has been described by Secretary of State George P. Shultz as a "moment of opportunity."

Burt, under questioning, said liaison officers such as Nicholson normally carry cameras but not firearms on "monitoring" missions in East Germany. Fourteen such U.S. military personnel are stationed in Potsdam, East Germany, to provide liaison with and monitor activities of the 440,000 Soviet troops there. A similar Soviet liaison force is in West Germany to monitor U.S. forces.

Nicholson held accreditation from Soviet military forces in East Germany and was traveling in a clearly marked U.S. vehicle, according to Burt, who displayed a map of East Germany spotted with dozens of "permanently restricted areas" where U.S. liaison teams are not permitted.

But Burt said Nicholson and Schatz had not penetrated the restricted zone at Ludwigslust, although they were very close to it.

Burt said that, while there have been incidents between the Soviets and Western military officials in East Germany, he was not aware of any recent increase in tension or incidents and had no report of earlier incidents in the area where Nicholson was shot.

Nicholson "was doing nothing except what we're entitled to do under the 1947 agreement," Reagan told reporters during a photo session at the White House. He called the killing "a tragedy that never should have happened."

Noting Reagan's subdued tone, a reporter called out, "There seems to be a lack of outrage on your part, sir."

The president responded, "A lack of outrage? No, you can't print what I'm thinking."

Earlier in the day, at a breakfast meeting with reporters, Reagan said, "We're resentful and feel it's an unwarranted tragedy."

Speakes said that Reagan attempted to telephone Nicholson's widow in Berlin to express his sorrow but that she was on her way to an airport and that the call would be tried again today.

Nicholson's body was taken to West Berlin and was being flown to West Germany last night, according to Burt.

Officials said Schatz was being debriefed in Germany about details of the shooting.