The United States joined today in a strong and unanimous Security Council condemnation of South Africa for its "premeditated, aggressive" raid against Lesotho last Thursday, in which 42 people were killed.

Diplomats called it the strongest condemnation of South Africa for which the United States has voted since the Reagan administration took office.

U.S. officials said they joined in the council verdict because the resolution had been modified and to underline the urgency of resolving the region's problems "through peaceful negotiation and conciliation."

They referred to talks between South Africa and Angolan officials about the territory of Namibia, which were held in Cape Verde just before the raid, and to Soviet-American negotiations in Moscow on guarantees for a settlement that would involve the withdrawal of Cuban troops from Angola.

The Americans said South Africa had been told of the U.S. stand that the Namibia talks should not be disrupted by incidents such as the Lesotho raid.

Today's resolution backed the U.S. position by declaring that "there are peaceful means to resolve international problems and . . . only these should be employed."

It called the raid a "flagrant violation of the sovereignty and territorial integrity" of Lesotho, and demanded that South Africa pay "full and adequate compensation for the damage to life and property."

It also reaffirmed Lesotho's right to give sanctuary to refugees from South Africa.

South Africa contends that the raid was a strike at members of the outlawed African National Congress who it said had planned terrorist raids across the border.

King Moshoeshoe II, the figurehead ruler of the landlocked nation, which is totally surrounded by South African territory, appeared before the council and claimed that the victims were refugees, not terrorists.

The Lesotho delegation had sought a compromise wording for the resolution, to win the votes of the United States and other Western countries. To achieve this, the claim that the attack was "unprovoked" was deleted at American insistence.

To avert any hint that sanctions might be applied against South Africa, a reference to generalized South African "aggression" against Lesotho (which is punishable by sanctions under the U.N. Charter), was changed to the more limited condemnation of South Africa's "aggressive act."

Possible broader ramifications of the Lesotho raid, which followed by a day South African talks with Angolan officials, came in a speech to the General Assembly yesterday by Angolan Ambassador Elisio de Figueiredo.

He charged that South Africa had "created" issues to block Namibian independence, among them "the totally unrelated issue of Cuban forces in Angola." Next, Figueiredo suggested, "South Africa will tie the issue of African National Congress activity to the Namibia issue; it has laid the groundwork by the recent massacre in Maseru."