The Soviet Union is stepping up support for its embattled Third World allies, apparently in response to increasing U.S. assistance to anticommunist insurgencies, according to Reagan administration officials and rebel sources.

These sources report that more sophisticated Soviet arms are being used in Afghanistan, that additional Soviet pilots and advisers are arriving in northern Ethiopia and that Cuban troops are moving to the front lines in Angola in preparation for new government offensives against the insurgents.

U.S. Soviet analysts are watching these developments closely for clues to any new directions in Soviet policy toward its Third World allies since Mikhail Gorbachev took power 14 months ago.

Of particular interest to these specialists is the Soviet leader's attitude toward the five regional conflicts -- Afghanistan, Angola, Cambodia, Ethiopia and Nicaragua -- that the Reagan administration has singled out as battlegrounds for challenging Soviet influence.

To date, these analysts note a continuing strong Soviet commitment to their closest allies. Moscow has just pledged additional military support for Libya, and already sent SA5 antiaircraft missiles. It has also promised Syria more arms and there are unconfirmed Israeli press reports that Moscow is about to give the Syrians the highly accurate SA23 ground-to-ground missile with a 250-mile range.

Despite this Soviet response to U.S. pressure, U.S. Soviet experts detect a reluctance by Moscow to take on any new burdens.

In the case of at least one ally, Mozambique, the Soviets seem hesitant to increase their aid significantly, even though the Marxist regime there is engaged in an uphill struggle against a spreading anticommunist insurgency and is dependent mainly on Soviet arms.

But this general lessening of Soviet interest in Third World affairs has not been evident in the nations where the superpowers have clients vying for power.

In Afghanistan, the Soviets have begun a new offensive against U.S.-backed rebels, increasing the size of their forces there by several thousand to about 120,000 and making wide-scale use for the first time of their fastest ground attack plane, the Su25, to avoid rebel missiles. They have introduced longer-range artillery to bombard mountain rebel strongholds, wire-guided rockets and "seismic mines" that explode through vibration, according to U.S. officials and rebel sources.

In Angola, the Soviets have replaced all the war materiel the army there lost last year in its offensive against South African- and U.S.-backed guerrilla forces. Top Cuban and Soviet military commanders have been involved in planning a new government bid to capture the headquarters of the rebel leader, Jonas Savimbi, in southeast Angola.

A new Angolan offensive against rebel strongholds in the south, with the participation of Cuban troops, began on May 27, according to spokesmen here for Savimbi's National Union for the Total Independence of Angola. This claim could not be independently confirmed.

After initiating a $15 million to $20 million covert aid program to anticommunist rebels in Angola late last year, the administration decided in March to send shoulder-fired Stinger antiaircraft missiles to Angola and Afghanistan -- the first U.S. arms to be introduced in either conflict.

In Ethiopia, rebels fighting for the independence of Eritrea Province say 4,000 to 5,000 additional Soviet technicians and pilots have recently arrived in the provincial capital of Asmara in preparation for the ninth Ethiopian attempt to seize their headquarters at Nakfa. A new Soviet-backed government offensive is expected to begin there late this month or early July, according to Bereket Habte Selassie, spokesman for the Eritrean People's Liberation Front at the United Nations.

Of the five major East-West regional conflicts mentioned repeatedly by President Reagan, only in Cambodia and Nicaragua have there been any signs of a lull in the fighting this year. Analysts say this is mostly because Soviet-supported governments in both countries made major military gains against their guerrilla opponents last year.

U.S. analysts say the Soviets are just beginning to devise a counterinsurgency strategy to deal with the U.S.-backed insurgencies.

While tactics have varied from country to country, one common Soviet objective for two years has been to invade the heartland of the insurgency in an attempt to occupy its headquarters, apparently in hopes of dealing it a knockout blow and demoralizing supporters. This tactic has been followed in Ethiopia, Afghanistan and Angola as well as Mozambique.

In Afghanistan, the Soviets have also tried to broaden the government to include noncommunist elements. But this tactic has not been tried in the other conflicts.

Despite this stepped-up Soviet commitment, administration analysts say they see no evidence the new Soviet leadership under Gorbachev has devised any special new "doctrine" to deal with the Reagan Doctrine of aid to anticommunist resistance movements.

"What seems to be happening is that they are trying not to respond to the Reagan Doctrine by finding a different doctrine but by pursuing their own long-term interests, just like they always were, in all five regional conflicts," said one defense analyst.

Administration analysts say Moscow has been in the throes of a full-scale reassessment of its relations with its Marxist allies for several years, and their interest in the Third World has waned since Gorbachev took office. Gorbachev, they note, scarcely made any reference to the Third World in his speech to the 27th Soviet Communist Party in February.

Francis Fukuyama, a former State Department policy planner now with the Rand Corporation, calls the Soviet policy "muscular consolidation" rather than retrenchment. In an article appearing this spring in Foreign Affairs, he summed up the Soviet attitude as one of "greater caution" in taking on new commitments and of concentrating on the problems of protecting established Soviet positions.

Fukuyama and other U.S. Soviet analysts also detect an increasing emphasis by Moscow on seeking closer ties with "regional influentials" such as Algeria, Mexico, India and the Philippines and on trying to establish diplomatic relations with the Arab monarchies of the Persian Gulf, most notably Saudi Arabia. Two of them, Oman and the United Arab Emirates, agreed to do so last fall.