Socialist Mozambique, whose wooing by the Reagan administration is widely viewed as America's one clear diplomatic success in southern Africa, is beginning to have second thoughts about its historic opening to the West.

There is a growing feeling inside the government of President Samora Machel that the United States has failed to deliver on promises of support it made two years ago when Machel took a big political risk by signing a nonaggression pact with neighboring, white-ruled South Africa.

Under the 1984 agreement, Maputo expelled hundreds of members of the African National Congress, the main guerrilla group battling the South African government. In return, Pretoria pledged to halt its support for diffuse bands of anti-Marxist rebels and mercenaries operating inside Mozambique.

The accord itself is in tatters due to alleged violations by the South African military, which officials here say has continued to provide antigovernment forces with arms and supplies despite the agreement.

They also declare that the United States has not applied maximum pressure on South Africa to honor the treaty.

"We understand that there is a process which will take time, but we can clearly state that the United States of America is not yet seen to be doing tangible things in Mozambique," said Foreign Minister Joaquim Chissano in a recent interview. "There's still a lot of hesitation. More can be done by the administration."

A key architect of the agreement with South Africa -- called the Nkomati pact, for the South African border town where it was signed -- Chissano stressed that his government would continue to honor the accord and that it still hoped Washington would fulfill its commitments. But analysts here say there are others in the ruling Politburo who are urging Machel to back away from the country's growing reliance on the West.

"Their main argument is that the United States is not being serious with Mozambique," said a Mozambican analyst here. "It's quite clear that after two years of Nkomati, the U.S. still hasn't made up its mind. And it's not clear what these guys want from us."

While Nkomati, on paper, was a bilateral accord between Maputo and Pretoria, it was part of a larger compact between Mozambique and the West. Around the time it was signed, Machel invited western aid agencies to set up shop in Maputo, joined the World Bank and the International Monetary Fund, and embarked on a series of reforms designed to revive the country's moribund economy.

In turn, the West offered the prospect of increased aid, new investment and assistance in fighting the rebels of the Mozambique National Resistance Movement, known here by the Portuguese acronym Renamo. The crowning event of the rapprochement with Washington was Machel's visit to the White House last September.

Western nations have supplied emergency food aid to help cope with the effects of a five-year drought, now abated, and the continuing war against Renamo.

But these countries have been unable or unwilling to supply the government's other key need -- arms and other military support against the rebels. As a result, Maputo remains heavily dependent on the Soviet Union and other East Bloc nations.

Since 1983, the United States has been Mozambique's largest food donor, supplying $73 million in grain and other staples and earmarking $36 million more for this year. But Washington has been less forthcoming with economic development assistance, in part because of congressional opposition.

Congress blocked plans to expand the modest aid program, making $5 million of aid conditional on the government's holding free elections -- regarded by this one-party state as an unacceptable demand.

The Gramm-Rudman balanced budget legislation also took a bite, leaving the program at $9.5 million, $3.5 million less than last year.

Administration efforts to supply a small amount of nonlethal military aid -- $4.6 million over two years -- also have been blocked in Congress. And American corporate investment, which was one of the carrots dangled in front of Machel two years ago, has been virtually nonexistent due to the war.

Mozambique has been similarly unsuccessful in obtaining military support from other western nations. Britain has initiated a small-scale training program for Mozambican officers, but Portugal, which once ruled Mozambique, has not responded to pleas for training and supplies.

Soviet diplomats were disappointed by Machel's signing of the Nkomati accord. They reportedly were not informed of the pact until the last minute. Nonetheless, Moscow has continued to be Maputo's main supplier of military hardware.

The Soviet Union last year provided an undisclosed number of military advisers, MiG aircraft and assault helicopters that are crucial to the government's fight against Renamo. When the rebels blew up Maputo's main ammunition depot last September, the government was compelled to ask Moscow for replacements. Moscow also provides 75 percent of Mozambique's oil supply, reportedly at concessionary rates.

Machel acknowledged this debt in a speech opening the People's Assembly in December, thanking the Soviet Union for the "enormous quantity" of weapons and other equipment it had sent. He also made a state visit to Moscow last month in which he requested more aid from Soviet leader Mikhail Gorbachev.

American diplomats, still seeking to salvage Nkomati, contend that they raise the question of compliance every time they meet with South African officials to discuss regional issues.

Appearing before a congressional panel last month, Secretary of State George P. Shultz called the treaty "an important marker, even though you can point to that accord and wonder if the South Africans, in particular, are keeping their side of the bargain fully. We know they aren't."

Nonetheless, Mozambican officials believe Washington is not tough enough with Pretoria. "They don't take a firm stance on the issues," said Chissano of the Americans.

"If they condemn the actions of the South Africans, they also appease them at the same time. They say that apartheid is bad but we have got to move slowly. Namibia must be independent but the Cubans must go from Angola, he added. "They want peace in Mozambique but the Mozambicans must be more cooperative. There's some sort of a lack of determination in what they are doing with South Africa."

Some Mozambican officials are also concerned that the United States is trying to push them into negotiations with the rebels.

When Frank Wisner, former deputy assistant secretary of state for Africa, was here in March, sources said he repeatedly tried to raise the issue in a private meeting with Machel. The Mozambican president, who has taken a hard line against such talks, ducked the question each time.

Mozambican unease was evident tow weeks ago when the state radio station ran a brief news item claiming that some senior U.S. officials, including Defense Secretary Caspar W. Weinberger and CIA Director William J. Casey, favored covert aid to Renamo along the same lines as the aid now being given to antigovernment forces in Angola. The U.S. Embassy issued a public denial and officials here seemed mollified.

But Chissano, noting that the State Department had been unable to persuade Congress to increase aid to Mozamibique, said he feared it could also prove unable to stop aid to the rebels.

"I don't know whether they will be able to resist the pressures of right-wingers in the Congress," he said.