The Defense Department plans to test a submarine-launched missile with 12 dummy nuclear warheads next month, two more than in previous tests, a move that arms control experts and congressional sources said yesterday could complicate reaching a U.S.-Soviet agreement to reduce strategic nuclear arms.

Critics of the planned test of the D5 missile charge that under the prospective agreement on strategic, or long-range, weapons each D5 missile might be counted as having 12 warheads, even though some of them will be deployed with fewer.

Rep. Les Aspin (D-Wis.), chairman of the House Armed Services Committee, recently wrote Secretary of Defense Caspar W. Weinberger that such a test "could have profound implications for U.S. strategic force planning" because it would lead to a misleading count of U.S. warheads.

In particular, Aspin said, it could result "in the United States deploying fewer warheads that it would {otherwise} be allowed" under the strategic arms agreement.

Aspin told Weinberger that he should defer the expanded missile test "until we have a clearer understanding of how it will mesh with our arms control policy." Weinberger responded that the test will not have adverse effects on the agreement and would go as planned.

Critics of the test plan said the Soviets could be regarded as having a military advantage if they deploy as many warheads as permitted under the prospective arms agreement, while the United States deploys fewer than permitted.

They also said that if each submarine-launched missile is counted as having 12 warheads instead of eight or 10, the United States would be forced to deploy fewer D5s, and perhaps fewer submarines to carry them. The D5 missile is carried by the Trident submarine, which would be difficult to attack in a preemptive Soviet strike.

"I have a hard time understanding why Weinberger made this decision, because it seems to run counter to sound arms control and defense policy," said Spurgeon Keeny, director of the private Arms Control Association.

Several officials said the test plan had been hotly disputed by the Navy because it could lead to deployment of fewer Trident submarines under a superpower arms agreement. But other officials said the problem could be averted if each superpower agreed to count the actual number of warheads on submarine missiles, rather than counting warheads as if each missile carried the maximum number used during test flights.

Some officials said Weinberger favored the "actual" counting approach, which would require intrusive on-site inspections to verify compliance with any agreement. But other administration officials oppose on-site inspections, saying the Soviets could add to warheads on submarine-launched missiles after any inspection.

The Pentagon held a Trident 2 test yesterday.