The FBI has been trying to expand its counterterrorist forces since last year but has been turned down twice by the White House, according to a member of the Senate Select Committee on Intelligence.

Sen. Lloyd Bentsen (D-Tex.) said he learned of the budgetary lid as a result of his inquiries about the adequacy of the bureau's resources.

"It's one of the most foolish things I've seen since I've been up here," Bentsen said. "Here you have terrorism on an increase around the world, and you know it's going to increase in the United States and we must fight it."

According to figures compiled by Bentsen and his staff and verified by other sources, the FBI had been seeking an $11 million increase over last year's $39.5 million counterterrorism budget to pay for 191 more agents, support personnel and related expenses.

The biggest chunk of the additional appropriation, about $5.7 million, would have been used to expand FBI counterterrorism task forces set up with local police in Boston, New York, Chicago and Washington since 1980 and to establish new ones in Newark, Los Angeles and San Francisco.

The rest of the money would have been devoted to strengthening the FBI's elite Hostage Rescue Team, now said to have about 50 agents, and broadening the scope of the five-year-old National Terrorism Research and Analyis Center based here.

The White House Office of Management and Budget rejected the proposals last fall in trimming a supplemental budget request for fiscal 1985 and again this year in ruling on the 1986 budget proposal before Congress.

Voicing alarm in a weekly videotaped report to Texas constituents, Bentsen said he regards the frugality as "just counterproductive" and said he would introduce legislation "to put that $11 million back."

Bentsen's press secretary, Jack DeVore, said the OMB limited the FBI counterterrorism budget to $39.8 million, a 1.1 percent increase in a year when, according to OMB projections, inflation is expected to be 4.4 percent.

DeVore said Bentsen learned about the $11 million trim in asking about the FBI's resources to cope with such counterintelligence problems as the alleged Walker spy ring. He said Bentsen was told that the counterintelligence budget was in good shape but that the counterterrorism program faced constraints.

OMB spokesman Edwin L. Dale Jr. declined to comment. "We don't discuss decisions made back at budget time," he said.

The FBI declined to voice public chagrin. Spokesman Tony Genakos said FBI Director William H. Webster is "supportive of the administration's request to Congress for fiscal 1986 in connection with our terrorist activity, and we're also grateful for the support we've received from the administration and Congress in combating terrorist operations."

Bentsen aide Jim Currie said, however, that the extra $11 million would "give the FBI greater ability to deal with domestic hijackings and hostage situations, to identify terrorist groups that are an outgrowth of or have an affiliation with foreign governments or movements and to be in a position to tell who these groups are affiliated with, where they are in the United States and what they are doing and planning to do."

The FBI's hostage rescue team was established in January 1983 and, Genakos said, is "a cohesive unit able to respond to highly sophisticated hostage situations. It gives the president and the attorney general a viable law enforcement alternative to the use of a military group for the resolution of a domestic incident."

The terrorism task forces, starting with that established in New York in April 1980, are teams of FBI agents and local police, usually housed in FBI quarters and designed "to make the most use of all the laws available" in concurrent investigations of terrorist crimes, plots and threats.

The Terrorism Research Center has been operated here since 1980 and consists of a computerized data bank that compiles information on known active terrorists in the United States and tries to determine "the potential threat of further terrorist activity," Genakos said. It is directly linked to most FBI field offices.

Bentsen is expected to offer an amendment giving the FBI the additional $11 million and, Devore said, will probably try to attach it to "the first handy appropriations bill that comes down the pike."

Little opposition is expected. "This may well be one of the easiest legislative victories in the history of man," DeVore said.