A senior Nicaraguan official said yesterday that the lengthening of airport runways disclosed by a U.S. intelligence briefing Tuesday was originally proposed by a U.S. government-financed study during the administration of ousted president Anastasio Somoza.

Jaime Wheelock, Nicaraguan minister of agriculture and one of the nine-member directorate of senior commanders with greatest authority in the country, also charged that the United States has "flagrantly violated" its international obligations by conducting overflights and taking photographs of military installations in that country.

Wheelock, in a meeting with Washington Post editors and reporters, disputed the interpretation and details of Nicaraguan military activity described in the intelligence briefing. "The 'evidence' which was produced is not proof of anything," he said.

According to Wheelock, lengthening of runways, one of the activities covered in detail by the briefing, was recommended by a U.S. study during the Somoza administration.

A spokesman for the Agency for International Development later confirmed that the 1975-76 study of Nicaraguan transportation needs, financed by about $1 million in U.S. funds, was conducted by Wilbur Smith & Associates, a U.S. consulting firm.

AID spokesman Rich Miller said the study recommended extension of runways in several airports to improve Nicaragua's transportation system.

However, he added that the study did not recommend construction of revetments for military aircraft, deployment of antiaircraft weaponry or training of Nicaraguan pilots on Soviet Mig warplanes, all of which were reported in the briefing.

No mention was made of revetments by Wheelock or in the documentary evidence of the U.S. study that he provided. Wheelock denied assertions in the U.S. briefing that 50 Nicaraguan pilots are being trained in Bulgaria and Cuba to fly Soviet Migs.

"No Nicaraguans . . . not a single Nicaraguan, is being trained to fly Migs," Wheelock said.

The public briefings Tuesday, presented by Bobby R. Inman, deputy director of the Central Intelligence Agency, and John T. Hughes, deputy director of the Defense Intelligence Agency, did not address evidence in U.S. hands of external control of guerrilla forces in El Salvador.

Secretary of State Alexander M. Haig Jr. said last week that "overwhelming and irrefutable evidence" of such control by non-Salvadorans has been presented to the intelligence oversight committees of Congress in highly classified briefings.

A "dry run" of Tuesday's briefing was presented last Friday to Haig, who said it needed improvement before being made public. Because of "a terrible problem of sources and methods," according to an administration official, little of the briefing as formulated dealt with outside intervention in El Salvador, the central issue being debated in Congress and the press.

It was decided, therefore, to delete all mention of this issue from Tuesday's revelations and to concentrate on Nicaraguan internal developments.

Officials are still promising a public session, perhaps tomorrow, to disclose information about outside direction of the Salvadoran guerrillas.

The growing expectation is that defectors or agents claiming to have firsthand knowledge will be produced at that time. Such testimony, according to official sources, is less sensitive than the communications intelligence that evidently is the basis for statements by Haig and some other officials.

Wheelock, who left here yesterday without success in his effort to meet Haig, depicted military activities in his country as defensive. He said the U.S. briefings had been deceptive in substance and prone to "excessive hysteria" in tone.

Nicaragua is being threatened by the United States verbally and in a reported plan of U.S. undercover actions, rather than the other way around, according to Wheelock.

He appeared to be as concerned about the indignity of having U.S. intelligence photos taken and displayed as by their substance. The briefings, as reported by the press, showed "a lack of respect and a great deal of exaggeration" on the part of U.S. officials, he charged.

The weapons shown by the aerial photographs, Wheelock said, "are far from sophisticated; in fact, they are obsolete."

Wheelock disputed the U.S. statement that Nicaragua is assembling the largest military force in Central America and is a threat to its neighbors.

Hughes, in the Tuesday briefing, did not repeat Haig's statement of last Thursday that Nicaragua has a 25,000-man army and a goal of doubling that. Instead, the DIA official said the Sandinistas' regular military force has grown "from a small initial force of 5,000 to a large active-duty army supplemented by even larger militia and reserve elements."

Hughes did not disclose a U.S. estimate for the regular army but put this force plus militia and reserves at "up to 70,000 men" and "the largest in Central America."

Wheelock said Guatemala has "a regular army of 35,000," which he said is "twice the size that Nicaragua has." In an interview last Saturday, Wheelock put Nicaragua's regular army at "not larger than Somoza had," about 14,000.

Wheelock denied that there is a plan for expansion of the army but said, "If we did not build up our defenses after what is happening now, we would be very irresponsible."