A high-level U.S. military team flew into Morocco today against a background of intensified appeals from King Hassan II for increased American support in his war against Libyan-backed guerrillas in the former Spanish Sahara.

The U.S. delegation, including 23 Pentagon and State Department officials, is headed by the assistant secretary of defense for international security affairs, Francis J. (Bing) West Jr. Although Morocco is only one stop among several Arab and black African nations, the talks here were interpreted as particularly important because of Morocco's request for upgraded diplomatic and military backing from the United States after a a jolting defeat Oct. 13 by Polisario guerrillas who routed a Moroccan garrison at the remote Saharan outpost of Guelta Zemmour.

The Guelta Zemmour setback was described by one diplomat here as "a very cold shower indeed" for Hassan's efforts to bring the six-year-old Sahara conflict to an internationally acceptable end without abandoning Morocco's claim to the disputed territory. In the battle, Moroccan authorities report, the Polisario guerrillas for the first time used T54 tanks and sophisticated ground-to-air missiles that escalated the conflict to a dangerous new level.

Although the Moroccan reports were greeted with reserve by Western military experts here, Hassan's government indeed apppears to have been shaken by the reversal. The monarch is said to be particularly upset by the loss of five planes -- a C130 Hercules, two Mirage F1s, an F5 and a Puma helicopter -- in fighting around Guelta Zemmour.

The losses were the largest in such a short time since the conflict began after Spain withdrew from its former colony in early 1976. In addition, they signaled a new danger to Morocco's ability to conduct air reconnaissance over the desolate desert tracks used by Polisario irregulars to infiltrate from Algeria and Mauritania.

As a result, Morocco's request for more military help from the United States centers on equipment that will permit Hassan's Saharan forces to reconnoiter the region without being exposed to the danger of being shot down by Polisario missiles or antiaircraft fire.

Moroccan and foreign press reports have spoken of possible use of Airborne Warning and Control System radar planes such as those dispatched to Egypt and sold to Saudi Arabia. Other sources dismiss these reports as wishful thinking, however, saying the Moroccan request has not progressed to such specifics and that, in any case, AWACS planes are unlikely to be part of the U.S. response.

At the same time, the Reagan administration appears determined to include Morocco on the list of U.S. friends who must receive concrete demonstrations of support in the face of Libyan pressure in Northern Africa and uncertainty following the assassination of President Anwar Sadat of Egypt. The new U.S. ambassador here, Joseph Verner Reed Jr., is known to be particularly eager to increase U.S. support for the king's military struggle in the Sahara and his economic struggle in the drought-stricken Moroccan countryside.

Secretary of State Alexander M. Haig Jr.'s expressions of concern at Reed's swearing-in in Washington were front-page news here several days and Haig's special itinerant ambassador, Gen. Vernon Walters, quietly passed through Morocco for a visit with Hassan last week.

Apparently with this in mind, Hassan has emphasized foreign involvement in the Guelta Zemmour battle. He complained that one of the three Polisario columns that overran the 2,000-man Moroccan garrison came from Mauritania and that the Moroccan Hercules was shot down by a SA6 ground-to-air missile operated by "non-African" technicians.

The monarch also pointed out that his agreement at last June's Organization of African Unity conference in Nairobi, Kenya, to a cease-fire and referendum in the Sahara included a tacit understanding that Mauritania and Algeria would cooperate in maintaining the truce. After first declaring that the Guelta Zemmour attack opened the way for canceling the deal, Hassan told an OAU envoy last week that he will continue preparations for the referendum to determine the future of the territory's estimated 100,000 inhabitants.

With Libyan-supplied Soviet weapons the main Polisario armament, Hassan also called in the Soviet charge d'affaires here to lodge a formal protest against Polisario's alleged use of the sophisticated missiles and T54 and T55 tanks.

Military experts here said Hassan's conclusion that Polisario troops used SA6 missiles grew mainly from orders that the C130 was to fly a reconnaissance mission at 18,000 feet, out of range of the shoulder-fired SA7 missiles long used by Polisario.

In fact, they noted, a less powerful SA9 missile also could have downed the plane at that altitude and, in any case, there is no reason Soviet advisers in Libya could not train Polisario guerrillas to fire SA6 missiles.

Moreover, foreign journalists taken in Moroccan Army helicopters to see proof of the Polisario's sophisticated new weaponry found only a row of shell casings and some armored vehicle tracks on a dusty road. There was no trace of the T54 tanks the Moroccan Army said it had destroyed in retaking the outpost during two days of fighting, they reported.

But whatever the actual weaponry, the operation cost Hassan a battalion's worth of equipment, five aircraft and what Moroccan authorities have described as a high number of casualties. More importantly, diplomatic sources pointed out, it raised questions about Moroccan ability to use air power to back its land forces and keep abreast of Polisario movements in the desert.

"That's a hell of a change, and it's reason for concern, because if they have it improved weaponry , the question is, 'what next?' " a Western military observer said.

It is this question that West and his team are expected to get from Hassan during their three days here. Ironically, it comes as Moroccan pilots are learning to fly six of 10 observation planes delivered only this summer by the United States in an earlier effort to improve the Moroccan Army's reconnaissance ability.

Diplomatic sources said the United States also is considering diplomatic help for Hassan, perhaps urging France to intervene with Algeria to reduce its support of Polisario forces, whose main headquarters lie at Tindouf in Algerian territory. Another possibility, they suggested, is to seek increased financial help for Hassan from Saudi Arabia.

The matter is considered especially urgent here because of Hassan's agreement to the referendum at Nairobi. The monarch's main opposition, the Socialist Union of Popular Forces, questioned the Nairobi arrangement in a statement last September -- whereupon its top leadership was arrested and put under house arrest in a remote town.