The Soviet Union and Angola concluded a new 10-year economic cooperation agreement and pledged themselves in a communique published today to further strengthen relations in other fields.

The two countries also signed a five-year trade, economic and technical cooperation accord during talks between Soviet Premier Nikolai Tikhonov and Lucio Lara, secretary general of the ruling Popular Movement for the Liberation of Angola and regarded as the second most powerful man in Angola, after President Jose Eduardo dos Santos.

Apart from increased economic assistance, the Soviets are believed to have promised Angola new military aid. Well-informed diplomatic sources said the discussions centered on the general review of the situation in southern Africa and Soviet fears that the Angolans may be lured into a settlement of the Namibian crisis at the expense of the Southwest Africa People's Organization (SWAPO), a nationalist force supported by Moscow.

Tikhonov, in a luncheon speech to the Angolan delegation, made the only public reference to the new Western initiative on Namibia when he attacked "a broad plot of international imperialism."

"The purpose of this plot," he said, was "to intimidate the people of the young state, to return it to the sphere of Western influence and to make it renounce support for the just struggle by the people of Namibia for its liberation."

Moscow's concerns apparently were fueled by indications that Angola was eager to establish relations with the United States and by recent contacts between high U.S. and Angolan officials, including talks in Paris last week between Angolan Foreign Minister Paulo Jorge and Chester Crocker, assistant secretary of state for African affairs.

Diplomatic observers here said the Soviets are concerned that Angola may be tempted to accept the new initiative on Namibia. It reportedly envisages a Zimbabwe-type settlement that would prevent an outright takeover of power by SWAPO and is also linked to reducing the presence of Cuban troops in Angola.

South Africa, black African states in the region and other interested parties are studying the proposals by the five-nation Western "contact group"--the United States, Canada, France, Britain and West Germany--on constitutional principles for Namibia when it gains independence from South Africa.

In his speech, as reported by the official Soviet news agency Tass, Lara said Washington was trying to link the presence of Cuban troops in Angola and independence for Namibia, but he asserted that "these are two entirely different problems."

Cuban troops, he said, "have been staying in Angola at the request of the Angolan government." This cooperation, he added, "is limited to the territory" of Angola.

But Lara said that "the peoples of Angola and Cuba will continue to develop new forms of cooperation in the interests of both countries." Some diplomats here suggested that this could be interpreted as an Angolan desire to broaden its relationship with Cuba while reducing the Cuban contingent there. It is estimated that there are about 15,000 Cubans in Angola.

U.S. analysts estimated late last year that there were fewer than 1,000 Soviet military advisers in Angola. After two Soviet colonels were killed and a warrant officer captured during a South African raid in September, Moscow acknowledged the presence of Soviet advisers but said their duties "do not go beyond the boundaries of technical advice and the training of Angolan national personnel."

According to Tass, the talks here passed in a "warm, friendly atmosphere."

Besides Lara, the Angolan delegation included Defense Minister Pedro Maria Tonha, Planning and Foreign Trade Minister Lopo do Nascimento, National Bank chief Jose Carlos Victor de Carvalho, Deputy Security Minister Antonio Meedes de Castro and two senior officials of the Central Committee of the ruling Popular Movement.

Angola has been experiencing an acute shortage of foreign exchange because of a drop in revenue from oil and diamond sales, which were expected to provide about 90 percent of the country's exports last year, according to official figures.

Moreover, Angola has contended that frequent South African military raids accross its southern border with Namibia have inflicted economic as well as military losses.

There was speculation here that Angola wants Soviet surface-to-air missiles that could severely reduce South Africa's air superiority.

It is hard to come by statistics that could give an idea of the scope of Soviet aid to Angola or the state of the Angolan economy. Tass mentioned that the Soviets would help build a 450,000-kilowatt hydroelectric station in Angola and provide it with machinery and assistance in fisheries and cotton growing.