In a potentially important breach of U.S. military security, a 25-year-old Air Force lieutenant who serves as a deputy crew commander for a Titan nuclear missile has been arrested for visiting the Soviet Embassy here without reporting the visits to his superiors, the Pentagon said last night.

Officials at Strategic Air Command headquarters in Omaha said 2nd Lt. Christopher M. Cooke of Richmond visited the embassy three times between last December and this month.

Cooke is "in pre-trial confinement" at McConnell Air Force Base near Wichita, Kan., the Pentagon said.

He is charged with "violating Air Force regulations which require reporting all contacts with representatives of a communist country," the pentagon said. The maximum penalty is two years at hard labor, forfeiture of pay and dismissal from the service.

The case also has been referred to the Justice Department for possible prosecution for other alleged offenses, the Pentagon sources said.

The Titan is the largest and oldest missile in the U.S. arsenal and packs the biggest atomic punch. Fifty-three of the huge weapons are scattered in underground silos throughout the Midwest and Southwest, including 17 at McConnell, where officials said Cooke has been stationed since last June.

SAC officials, in a telephone interview last night, said Cooke's duties included monitoring the missiles' alert status and reacting to authentic messages involving emergency war-order preparation and execution.

The young officier thus is familiar with procedures and authenticating codes by which the United States would fire such missiles in retaliation. He also would be familiar with the way in which the Titan is kept on alert.

Cooke was serving as one of two officers who, in effect, would turn the key to launch a missile if the proper coded instructions from the president were received.

SAC officers said the episode was "a very serious matter, serious enough to confine the officer and keep him under guard" and to take "appropriate actions to guard national security." Pentagon officials said a check disclosed that no documents to which Cooke has access were missing.

There was no indication of why Cooke went to the embassy or what, if anything, he may have told the Soviets. There also was no indication of whether the Soviets had initiated the contact. A spokesman for the Soviet Embassy could not be reached for comment.

United Press International reported that Mrs. Richard C. Cooke, the lieutenant's mother, said the Air Force has been holding her son in protective custody in Kansas and at Langley Air Force Base in Hampton, Va., for almost a month. She also contended that the Air Force agreed to give her son an honorable discharge and drop the matter.

[The Air Force hasn't let me know anything. We've been sitting here just waiting," she said. Cooke did not tell her if he was making contact with the Soviets, she said.]

Although they said they could not be certain, the SAC officers said they coulc not recall another case of a missile officer in such a key role being involved in a similar situation.

Cooke, who is single, was graduated from Old Dominion University in Norfolk and did graduate work at the College of William & Mary in Williamsburg. He entered the Air Force in December 1979.

It was believed to be the first such case of government employe being accused of unauthorized contacts with a representative of a communist country since former CIA agent David Barnett pleaded guilty last October to spying for the Soviet Union.

In the last such military case, Lee Eugene Madson, a Navy security guard, was sentenced to eight years in prison in October 1979 after he walked out of the Pentagon with top-secret documents stuffed in his trousers. He pleaded guilty to an espionage charge.

In December 1978, a former low-level CIA employee, William Kampiles, was convicted of selling a top-secret spy satellite manual to the Soviets in what was widely viewed as a major breach of national security. The Kampiles case suggested how successful the Soviets had been in making contact with persons with access to important U.S. information.

The Titans, which date to the early 1960s, are in service at McConnell, Little Rock Air Force Base, Ark., and Davis-Monthan Air Force Base near Tucson.

The missiles has a single warhead rated at a nuclear explosive power equivalent to nine million tons of TNT. The vast bulk of the U.S. land-based ICBM force if composed of 1,000 Minuteman missiles, more than half of which have three warheads each with much less explosive power than the Titan.