The Soviet Union has agreed to sell $1.6 billion in weapons to India in a move that strengthens the already close ties between the countries and is likely to increase Soviet influence in South Asia.

An Indian government spokesman said yesterday that the weapons agreement, the largest New Delhi has ever concluded, was initialed in Moscow last week. India has 17 years to pay for the arms at an annual interest rate of 2.5 percent, terms considered extremely concessionary.

U.S. analysts viewed the move as a Kremlin effort to consolidate its position in the region after the Soviet invasion of Afghanistan.

Although the Indian spokesman refused to disclose the weapons involved, news services quoted Defense Ministry officials as saying they include an unspecified number of missile-equipped patrol boats, air-to-air and surface-to-surface missiles, heavy T72 tanks, antitank missile systems and electronic equipment.

These officials said that the purchase of 100 tanks will be followed by production of about 600 more under Soviet license in India. The Soviet Union has equipped the Indian Air Force with Mig21 jets, which now are produced under similar licensing arrangements.

Analysts here expressed surpise at the deal, which is substantially larger than Soviet arms transfers to India in the aftermath of the 1971 Indo-Pakistani war.

Moscow supplied India with $1.1 billion in various types of weapons between 1973 and 1977, a period when the Indians sought to revitalize their armed forces. At that time, the Soviet credits carried a 2 percent annual interest rate and were repayable over 10 years.

The strengthening of India's armed forces was expected to cause increased nervousness in neighboring Pakistan, whose leaders rejected a $400 million U.S. aid offer following the invasion of Afghanistan.

India and Pakistan have fought three wars since they became independent 32 years ago. Western analysts describe their relationship as one of "mutual paranoia," with Pakistan allied to China and the United States while India has been linked to Moscow.

According to these analysts, Moscow's decision to help rearm the Indians is based in large measure on its containment policy toward China. India has been the cornerstone of this policy, particularly since a Chinese invasion of India in 1962 helped solidify Soviet-Indian relations.

The decision of Prime Minister Indira Gandhi to conclude the long-term weapons deal with Moscow came after the Carter administration earlier this month relented on its anti-nuclear proliferation policy and indicated willingness to sell India 38 tons of enriched uranium fuel. The Nuclear Regulatory Commission, however, has opposed the shipment to India because New Delhi has not signed a treaty prohibiting the spread of nuclear weapons, and the U.S. Congress could block the offer. t

The Press Trust of India disclosed yesterday that India received a first installment of 27.2 tons of Soviet heavy water for Indian nuclear plants under an agreement signed in Moscow last February. In the agreement the Soviet Union promised to supply a total of 226 tons of heavy water.

Indian government spokesman yesterday sought to play down the new arms deal, saying it was part of "the normal pattern" of cooperation between the two countries.

They also pointed out that negotiations on the weapons procurement were started last fall by the government of former prime minister Morarji Desai. But analysts here said Desai had tried to diversify India's weapons purchases in an effort to reduce dependence on the Soviet Union.

Desai concluded a major arms ageeement in 1978 for the purchase of 40 Anglo-French Jaguar aircraft and manufacture of another 110 under license.

In Moscow, the announcement of the deal caught Western diplomats by surprise since they were not aware of a visit there by an Indian military delegation. Analysts recalled that Gandhi maintained very close ties with Moscow during her previous tenure and that her return to power in January restored cordial ties between the two countries.

The agreement was initialed by S. S. Siddhu, secretary of the Defense Ministry, who held talks in Moscow last week with Soviet Defense Minister Dmitri Ustinov and the chefs of staff of the Soviet Army, Navy and Air Force.

Indian defense experts yesterday defended the decision to purchase the Soviet T72 tank over West German Leopards and British Chieftains on grounds that the Soviet weapons proved more suitable to the Indian terrain.

But Western defense specialists suggested that concessionary Soviet credits may have played a key role in the Indian decision.

One of these specialists said India's decision to draw closer to the Soviet Union "at the time the Soviets are nibbling at parts of the region" would increase instability in southern Asia and probably prompt Pakistan to strengthen its military forces.

There were suggestions that Moscow's concessionary credits were in part a reward to India for its exceptionally mild criticism of the Afghan invasion.