The Mig 23 warplanes recently supplied to Cuba by the Soviet Union show no signs of having a nuclear weapons delivery capability and do not appear to be a threat to U.S. security, a senior Defense Department official said yesterday.

The official, who spoke to reporters on condition that he not be identified, said the limited number of planes supplied is a factor in his judgment of their military importance. U.S. intelligence has reported that less than one squadron of 16 Mig 23s has been acquired by the Cubans, according to officials.

The unstated implication of the defense official's remarks, as well as of the comments by President Carter to his news conference Thursday, is that the Mig 23s are not a military problem so long as their number and capabilities are restricted. Administration sources said the United States is carrying on still-incomplete discussions with the Soviet Union in an attempt to define more clearly what level of weaponry in Cuba is acceptable here.

The Soviet Union long ago supplied Cuba with Mig 21 warplanes, which are primarily air defense aircraft but have some air-to-ground capability, the defense official said. Some of the more advanced Mig 23s, which began to arrive in small numbers late this spring, have a conventional weapons ground attack capability, the official said.

The 1962 U.S.-Soviet understanding reached at the end of the Cuban missile crisis is not precise, according to American officials and students of the period, but in general it bans the Soviet placement of offensive weapons in Cuba. The line between offensive and defensive weapons in the high technology age is difficult to establish, however.

Both Carter and the defense official said there is no evidence that nuclear weapons are in Cuba. While any aircraft could be adapted to carry a limited number of nuclear weapons, special characteristics are required to give a plane nuclear delivery capability in an effective military sense, according to experts.

Regarding Iran, an area of great importance to the United States on the border of the Soviet Union, the defense official said a shift to a regime unfriendly to the West would have very serious military consequences.

The official refused to discuss U.S. contingency plans for intervention in Iran, but noted that the present turmoil there was not principally due to outside military threats. The potential of military action by the Soviet Union is neutralized to some degree by U.S. statements and capabilities, and potential U.S. action is neutralized in part by Soviet statements and capabilities, the official noted.

In recent days Soviet leader Leonid Brezhnev has warned against any U.S. military intervention in Iran. President Carter, saying the United States has no such intention, has cautioned the Soviets against intervention there.

High officials of the Carter administration, including those with top defense responsibility, made several statements in past months emphasizing the importance of the Persian Gulf and suggesting that the United States would dispatch military forces to the area in response to a Soviet or Soviet-backed threat.

In present circumstances, however, neither the military power of the United States nor the billions of dollars in sophisticated U.S. military equipment purchased by the shah is of much effect. Iran's current problems, the defense official said, present striking evidence that large military forces will not ensure internal security or the coherence of a society.