The Reagan administration has apparently rejected a Democratic proposal, endorsed by the Soviet Union, for a joint humanitarian relief operation in war-torn Mozambique, where an estimated 4.5 million people are suffering from food shortages, U.S. officials said.

The proposal, which would involve U.S. and Soviet coordination in distributing mostly U.S.-supplied grain in Mozambique, was presented to Soviet leader Mikhail Gorbachev by House Speaker Jim Wright (D-Tex.) and Rep. Mickey Leland (D-Tex.), chairman of the House Select Committee on Hunger, in April during a visit to Moscow.

The United States was to provide grain and the Soviet Union would provide planes, helicopters, trucks and ships, Wright said. "At each delivery point, there could be a Soviet and American person and trucks used with the insignia of both countries," he added.

Gorbachev found the proposal "very interesting" and proposed further talks, the speaker said. Wright said he mentioned the proposal to Secretary of State George P. Shultz, whose reaction was "not what I'd call an expression of wild enthusiasm, but he didn't say 'Don't do that.' "

Wright said he met last month with Assistant Secretary of State Chester A. Crocker and was told that the administration preferred working through the special United Nations disaster relief office recently opened in the Mozambique capital of Maputo.

Although the proposal is officially described as under consideration, several U.S. officials said they believe it is highly unlikely that the Reagan administration would accept a hand-in-hand partnership agreement with Moscow in a nation where a Marxist government is battling a self-described anticommunist insurrection.

Despite the leftist leanings of the government in Maputo, the U.S. government has strongly supported it.

The United States has pledged $75 million in relief aid for Mozambique, mostly in the form of nearly 200,000 tons of grain. The Soviet commitment thus far is less than $300,000, U.S. officials said.