The conservative prime minister of this small kingdom surrounded by South Africa today accused the Reagan administration of encouraging aggression by Pretoria against his country.

Chief Leabua Jonathan told reporters that while the United States had promised diplomatic help in easing South African pressure on his government, "instead I think the Reagan administration is by their recent actions encouraging the South Africans to destabilize us."

Jonathan accused Pretoria of supporting guerrillas and invoking economic sanctions to force him to evict South African refugees opposed to white rule there and to compel him to grant diplomatic recognition to the homelands being established for blacks within South Africa. In response, Jonathan said he plans to open diplomatic relations with Soviet Bloc nations and with China and North Korea in a major policy shift.

"I can never be a communist," said the prime minister in a rare interview with foreign journalists at his mountaintop home here. "I am a Christian and this is a Christian country. But we are out to establish friendships all over the world because we realize the danger that one day we will be strangled by South Africa."

Jonathan joins a lengthening list of black African leaders who have attacked the Reagan administration's policy of "constructive engagement" with South Africa because they say it has given Pretoria free rein to undermine its black neighbors. Administration officials say the policy is designed to encourage peaceful reform within South Africa and rapprochement between the white-ruled government and its neighbors.

Lesotho, whose economy is heavily dependent on South Africa for jobs and investment, has been under increasing pressure for the past year from Pretoria, which contends that the insurgent African National Congress uses Lesotho as a base for launching attacks on South Africa. ANC officials here deny the charge.

The South African campaign began with a commando raid on refugee houses in Maseru, the Lesotho capital, Dec. 9 in which 42 persons were killed. It turned to economic measures in May following ANC bomb attacks in Pretoria and Bloemfontein.

Lesothan officials say South Africa has blocked Lesotho residents from entering South Africa, held up supplies and even prevented treatment for the sick. Foreign Minister Evaristus Sekhonyana said today three persons had died waiting at the border for permission to travel to South African hospitals.

Jonathan, whose house is less than a mile from the border, said South Africa had trained and financed members of the antigovernment Lesotho Liberation Army--a charge Pretoria has denied strongly--and said the insurgents had attempted at least four attacks on his home in recent months.

"They allow the guerrillas to use their territory as a springboard for attacks against us," said the prime minister. Lesotho receives more than $19 million in U.S. aid annually, and Jonathan said he was grateful for the money. But he added that the United States was the only country that could convince South Africa to ease its pressure on his government.

"I know that Washington can do a lot to persuade South Africa to resist from giving us unnecessary trouble," he said. "But I'm sorry to say it looks as though Washington is not doing as much as we would have expected it to do."

There are an estimated 11,000 refugees in Lesotho, and the United Nations High Commissioner for Refugees says 30 to 40 more trickle in each month. South Africa recently presented a list of 68 it said it wanted evicted from the county as security threats. Lesotho said this month that only 22 of those are actually in the country and all have volunteered to leave.

The growing hostility between Jonthan's government and Pretoria is a marked change from a decade ago when Jonathan was considered Pretoria's foremost black ally. Jonathan today recalled the good relations he enjoyed with the late South African prime minister Hendrik Verwoerd, who during his term in the early 1960s was an architect of apartheid, South Africa's system of racial segregation.

"I was the first black leader to meet with him," said Jonathan. "We could agree to disagree on that one issue."

But Jonathan said the present government of Prime Minister Pieter W. Botha was attempting to make Lesotho a "scapegoat" for South Africa's internal problems. He said Pretoria had miscalculated badly in its hope that Lesotho would recognize the homelands.

"They thought we would be the first to recognize their Transkei one of the homelands , but to their disappointment they found out that they were wrong," said Jonathan. He added that Lesotho would never recognize any homeland "for the simple reason that we would be recognizing apartheid through the back door."