A U.S. official offered two weeks ago to send a transport plane into Grenada to take out former prime minister Maurice Bishop if he was released from house arrest, Barbados Prime Minister Tom Adams said today.

Adams said the U.S. offer appeared to cover both a release of Bishop by the Revolutionary Military Council, which arrested him on Oct. 14, or Bishop's forcible freeing by a raid being contemplated by several Caribbean states.

"It wasn't specified that they would only let Mr. Bishop get aboard the aircraft with the consent" of the Revolutionary Military Council under Gen. Hudson Austin, Adams said in an interview today. Adams refused to identify the U.S. official by name or rank.

Last Wednesday, Bishop and at least five of his Cabinet ministers and aides were killed after Grenadan demonstrators freed him from house arrest. The deaths led to Tuesday morning's invasion by U.S. and Caribbean forces.

It was not clear why the U.S. sought a role in freeing the 39-year-old Bishop, who had drawn strong condemnation from Washington for forging close ties with Cuba and the Soviet Union after he took power in a coup of his own in 1979.

In Washington, a State Department spokesman, told of Adams' comment, replied, "We're still looking into that," Washington Post staff writer Patrick Tyler reported.

Diplomatic sources have said the United States had been pressuring Grenada's island neighbors, most of them parliamentary democracies with close links to the West, to impose economic and military sanctions against Grenada in the months before Bishop's arrest.

However, a visit by Bishop to Washington last June and a reshuffling of his Cabinet were seen by some as moves toward moderation. Western diplomats generally believe Austin is a harder-line Marxist than Bishop.

Speculation here was that the United States was concerned that Bishop's continuing detention would lead him to request Cuban intervention to oust Austin. The United States may have believed that with Bishop safe in a third country, pressure for a Cuban operation would wane.

In a television address last night, Adams said that on Oct. 15, the Barbados Ministry of Security and Defense told him it had "been approached by a United States official about the prospect of rescuing Maurice Bishop from his captors and had been made an offer of transport."

Adams said in an interview today that Barbados officials, wondering if this was an offer of helicopters to carry in a raiding party, sought clarification. U.S. officials said the offer was for one transport plane.

"I heard nothing to suggest that if Mr. Bishop was being secretly brought out of Grenada they would refuse it to be used," Adams said today. Adams' 22-minute address also provided a further account of the negotiations that led to the decision to stage a surprise attack on Grenada Tuesday by forces backed by gunships, naval artillery and jet bombers.

Talk of securing Bishop's release by force or negotiation began immediately after the news of his arrest. "Whatever our difference in the past, Mr. Bishop deserved the support of the Caribbean governments," Adams said.

The day Bishop was killed, Oct. 19, the Barbados Cabinet agreed in an emergency session to mount a rescue raid and seek the aid of other Caribbean states and "larger, non-Caribbean countries with the resources necessary to carry out such an intricate operation," Adams said.

Bishop's death occurred as the Barbados Cabinet was meeting, and afterward, talk shifted to full-scale military intervention.

Last Thursday, Adams said, he was telephoned by Prime Minister John Compton of St. Lucia, who firmly proposed raising a multinational force. Later that day the Barbados Cabinet decided to support that idea.

On Friday, Adams said, he informed U.S. Ambassador Milan Bish and British High Commissioner Giles Bullard that intervention was being contemplated. Bish "undertook to convey the facts to President Reagan while awaiting a formal request, should one be issued."

Friday evening, defense ministers of the Organization of Eastern Caribbean States met in Barbados and agreed to invoke Article 8 of their Treaty of Association and request outside intervention, Adams said. Troop numbers were agreed on and formal planning began.

Also on Friday evening, Adams, Prime Minister Eugenia Charles of Dominica and Prime Minister Edward Seaga of Jamaica, which is not part of OECS, jointly requested U.S. participation from Bish.

Over the weekend, a U.S. naval task force headed for Grenada and U.S. Marine officers began planning the operation with their Caribbean counterparts. But Adams was told repeatedly that that United States had not yet made a final decision to join in the operation.

On Saturday Adams made a "fully formal verbal request" to the British high commissioner for the United States to take part in the operation.

On Saturday and Sunday, leaders from the Caribbean Common Market, a larger group than OECS, met in Trinidad to consider the Grenada question. By an 11 to 1 vote, the group agreed to apply sanctions against Grenada but did not formally call for intervention.

Adams said he was officially informed that the United States would participate only on Monday evening, when Bish arrived at his residence at 8:10 p.m. and read aloud a letter of acceptance from Reagan.