A new Soviet intercontinental-range ballistic missile, or ICBM, failed its first flight test about six weeks ago, according to U.S. officials.

Officials say the missile was launched from the Russian missile test center at Plesetsk on Oct. 26. Indications here are that the missile's first-stage rocket motor failed and the flight ended quickly.

Despite the failure, U.S. analysts believe the start of the flight test program is significant because the new weapon uses solid, rather than liquid, fuel for its rocket motors.

The solid fuel makes such weapons easier for ground crews to handle. More important, it makes the missiles themselves more reliable and thus would give Soviet commanders a higher percentage than before of missiles that are always ready to fire. Solid-fuel missiles are also said to be easier to protect in underground silos against the blast of attacking enemy missiles.

Virtually the entire Soviet ICBM arsenal of about 1,398 land-based missiles is now composed of liquid-fueled weapons, including all of Moscow's latest and most menacing weapons.

All 1,000 U.S. land-based Minutemen ICBMs, on the other hand, are solid-fueled. The only liquid-fueled U.S. missiles are 51 aging Titan II missiles that are now being withdrawn from service, in part because of recent accidents.

The Soviet test came as no surprise. Moscow has had several new missiles in development. But the start of the new flight-test program, analysts here believe, indicates that Moscow will probably begin modernizing its existing force of SS17 and perhaps SS19 missiles with a solid-fueled version.

Under the second strategic arms limitation treaty (SALT II) signed by presidents Jimmy Carter and Leonid I. Brezhnev in 1979, each side "may flight test and deploy one new type of light ICBM." Although the United States never ratified that agreement, the Reagan administration is abiding by it.

Sources here say that, in accordance with SALT II, Soviet Ambassador Anatoliy F. Dobrynin informed the State Department of the missile test shortly after it took place and reportedly described it as the one new missile that Moscow is allowed under the SALT II provisions.

The new Soviet test could figure in the developing debate on Capitol Hill over the administration's proposed MX missile program.

The Soviets have 150 SS17 missiles, each of which carries four individual nuclear warheads. There are 300 SS19s, each carrying six warheads. The SS19s, along with the still-larger SS18s, are considered among the most threatening to the U.S. land-based missile force.

Under SALT II, both the SS17 and SS19 are classified as "light" missiles, the classification that Moscow can modernize with one new missile. It is a classification that many American critics of SALT II believe does not properly describe the striking power of the Soviet weapons. Officials here are privately describing the new missile's size as medium to heavy. Because the test failed so quickly, it is not known how many warheads the new missile carried.

Sources say the Soviets coded the electronic signals coming from the missile, which also diminishes the information U.S. monitoring devices can gather.

The Russians have had trouble developing solid-fueled missiles for some time and may still be having trouble. The only other solid-fueled ICBMs in the Soviet arsenal are about 60 smaller SS13 missiles deployed in the late 1960s. This was Moscow's first effort at solid-fueled, long-range missiles, and western experts never rated the SS13 highly. Another smaller, solid-fuel missile, the SS16, was tested in 1975 but never deployed.

Since then, however, Moscow has used solid fuel in its SS20 intermediate-range missiles meant for use against Western Europe and China.