The United States, in an unusually swift and concrete response to requests from an endangered ally, promised today to provide Thailand with new and speedier aid in the light of cross-border incursions from Vietnam.

A list of hastily authorized U.S. actions, plus additional prospective measures still being formulated in Washington, was delivered this morning to Thai Foreign Minister Siddhi Savetsila by Secretary of State Edmund Muskie, who had arrived here only four hours earlier from Ankara, Turkey.

Thai requests for assistance were made through diplomatic channels Wednesday, following two days of heavy fighting with Vietnamese forces near the Thai-Cambodian border. U.S. sources said the decisions to provide the aid were made in Washington after strong messages from Muskie dispatched from his aircraft during the 12-hour flight from Turkey to East Asia.

Specifically, the United States promised to provide a $2 million grant for relief of Thais displaced by the Vietnamese military action; to speed up the delivery of 35 M48 tanks, small arms, ammunition, howitzers and recoilless rifles previously ordered by Thailand, and to make every effort to ease payment terms for military sales loans to Thailand and provide additional loans to finance future purchases.

American officials saw the new assistance as of symbolic as well as practical importance in reassuring the Thais that U.S. support is more than rhetorical in time of challenge.

The Thai foreign minister, first in a lineup of senior Asian diplomats to confer with Muskie in connection with the meeting here of the Association of Southeast Asian Nations (ASEAN), expressed public appreciation for the U.S. "quick response."

State Department spokesman John Trattner said various Asian diplomats who met Muskie today expressed "a very high level of concern" about the military confrontation between Thailand and Vietnam. The speeches and resolutions of this political conference of noncommunist Asian states included strong condemnation of Vietnam and verbal support for the Thais, but no announcement of tangible assistance.

Given the tense circumstances arising from the Vietnamese incursions, American officials considered it extremely fortunate that Muskie had decided in the face of contrary advice several weeks ago to travel here to "show the flag" and demonstrate U.S. solidarity with the annual ASEAN foreign ministers' meeting.

President Carter initially opposed the trip, as did most of Muskie's State Department aides, sources said. But the new secretary of state decided the trip could be an important gesture and obtained Carter's approval -- all long before the sudden clashes at the Thai border.

A U.S. official who briefed reporters on the border incidents, as pieced together after several days of study, said two companies or more of Vietnamese troops were "clearly in Thailand" in their military operations Monday.

The Vietnamese troops clashed with a Thai force on the ground, killing 15 Thai soldiers and destroying an armored car, according to the official. Both sides summoned air and artillery support, which led to substantial civilian casualties as a long-range duel continued Monday and Tuesday, he said.

Total casualties in the fighting were 24 Thai military killed, a number of Vietnamese dead -- which the Thais variously have cited as 31 and 75 -- and a "guess" of about 1,000 civilians, mostly refugees from Cambodia, killed or wounded, according to the U.S. official.

The U.S. official, who did not permit use of his name, said Vietnamese bodies were recovered inside Thailand. He cited this as clear proof that Thai territory had been invaded.

Vietnamese Foreign Minister Nguyen Co Thach, in a meeting in Bangkok Wednesday with U.S. Ambassador Morton Abramowitz, denied that Vietnamese troops crossed into Thailand, according to the U.S. official. Abramowitz met Thach at U.S. request to deliver a message of concern and disapproval of the border incidents.

In Washington, Soviet Ambassador Anatoliy Dobrynin was called to the State Department to receive a similar message, on the theory that Moscow can restrain or at least influence its Vietnamese allies.

In a public statement today in Hanoi, the Vietnamese Foreign Ministry described as "lies" the reports that Vietnamese forces penetrated Thai territory. The statement called the reports a maneuver "to shift opinion from the true cause of the current situation at the frontier and to camouflage the collusion between China and the United States, which, with help from Thailand, are intensifying their hostile activities" against Vietnamese-backed Cambodia.

The U.S. official declined to state categorically the motive behind the Vietnamese action. He said "it would appear" that Vietnam wanted to close the Thai-Cambodian border to block the continued availability of food and supplies to Cambodian insurgents fighting against Vietnam. The official said the new Thai program of arranging the return of Cambodian refugees, including those of military age likely to oppose Vietnamese rule in Cambodia, may also have been a factor in a decision to hit the border area.