In a decision meant to keep old friend Morocco happy but which could make new friend Algeria angry, the Reagan administration plans to give preliminary notice to Congress this week that it wants to sell 108 U.S. M60 tanks to Morocco.

Morocco is a longtime ally of the United States. For the past four years, however, the government of King Hassan has ben engaged in a controversial and inconclusive war for control of the Western Sahara region against Polisario Front guerrillas backed by Algeria, the country which did so much to gain freedom for the American hostages in Iran and a country to which many top officials here say the United States owes a big debt of gratitude.

According to State Department officials, the Moroccan request for the tanks was made last July, well before Algeria was involved as an intermediary in the hostage negotiations. The United States has been supplying arms, primarily defensive, to Morocco for some 20 years, and the tank request came on the heels of a Carter administration decision in March 1980 to sell it about $232 million worth of reconnaissance planes and helicopters.

Secretary of State Alexander M. Haig Jr., according to State Department sources, went ahead last Friday and allowed the transfer to Morocco under the original schedule of the first two of the six OV10 reconnaissance planes worked out in the March deal.

The tank request had first run into some internal opposition within the Carter administration. Then later, as Algeria entered the hostage talks, Deputy Secretary of State Warren Christopher reportedly warned the Carter White House -- after the November elections -- to put the tank deal aside and let the new administration decide how to handle it.

Although State Department spokesmen would neither confirm nor deny it, other officials said Haig has now decided to go ahead with the tank contract and would soon give Congress an informal 20-day notice as a prelude to the formal 30-day notification period required for such actions. Officials said the tanks would not be delivered for about three years, would not be effective weapons for the Western Sahara, and that Morocco has said privately it has no intention of using the tanks there.

Rather, U.S. officials say, the new weapons would be meant for conventional Moroccan defense in its own, rather than disputed, territory. While some U.S. officials say Algeria is likely to be "a little upset," they note that the Algiers government now has a four-or five-to-one advantage in tanks over neighboring Morocco so tahat the new weapons should not destabilize the situation between the two countries.

Though the pro-western regime in Morrocco has been leading moderate among Arab states, the decisions about supplying arms to the government in Rabat have increasingly become a foreign policy problem for Washington, which also would like to improve relations with other nations in the region. In his recent confirmation hearings in the Senate, Haig described U.S. relations with Morocco as "longstanding and historically cordial."