BUENOS AIRES, Argentina (AP) _ The Soviet Union, Argentina's largest trading partner in recent years, plans to meet grain purchase commitments and comply with an accord on fishing rights in Argentine waters, its ambassador said Thursday.

Last July, Moscow surprised Argentina by announcing it would not meet the 1986 purchase quota under a five-year grain trade accord, and instead would purchase rough grain from other markets, principally France.

Grain, Argentina's largest export commodity, has been under severe price pressure due to excess supplies on the world market.

Soviet Ambassador Oleg Kvasov said at a news conference that his country still planned to purchase 4.5 million tons of rough grain through 1990, as called for in an accord signed last January.

Although precise figures are not yet available, total Soviet purchases for last year were being projected at $400 million dollars, down from $1.35 billion two years earlier.

In Washington on Thursday, Agriculture Secretary Richard E. Lyng said the Soviets are virtually ignoring a long-term agreement requiring Moscow to buy minimum amounts of U.S. grain each year.

So far, no U.S. grain has been purchased by the Soviets for delivery in 1986-87. Last year's sales included 152,600 tons of wheat and 6.8 million tons of corn.

Under the five-year agreement, which runs through Sept. 30, 1988, Moscow is supposed to buy at least 9 million tons a year, including a minimum of 4 million each of wheat and corn.

The Soviets are pressuring Argentina to purchase more Soviet goods, especially heavy equipment, to reduce a large trade imbalance. Argentine imports from the Soviet Union were about $70 million in 1986, according to preliminary Argentine government estimates.

Kvasov said the Soviets also planned to comply with a new fishing accord giving them limited rights to trawl within Argentine territorial waters. However, he noted that the accord, approved by Congress in December, had not yet received final presidential approval.