President Reagan said yesterday that the problem of hunger in the United States is caused by "a lack of knowledge" about where to obtain help and added, "I don't believe there is anyone going hungry in America simply by reason of denial or lack of ability to feed them."

Numerous studies have reported that the nation's hunger problem is worsening, and a study released this week found that an administration effort to target food-stamp benefits to the "truly needy" is "a failure."

Speaking to 117 students from around the country, Reagan was asked why the administration has sent military aid to foreign nations rather than "taking care of people at home."

He defended the aid, then said that "where there is hunger . . . you have to determine that that is probably because of a lack of knowledge on the part of the people as to what things are available."

He added that there has been "about a three-times increase in private charity" nationwide and asserted that "much of that is in providing programs of food, in school lunch programs and so forth." He said that, with government and private programs, the reason for people going hungry is "not knowing where or how to get this help."

Critics responded by blaming Reagan's policies.

"The president announced today that hunger in this country is caused by ignorance," Sen. Edward M. Kennedy (D-Mass.) said. "In a sense, the president is correct -- hunger is caused by the ignorance of those who do not see the suffering of millions of Americans . . . . This did not happen because millions of people suddenly became ignorant; it happened because they or their parents cannot find work and the safety net was cut away."

J. Larry Brown of the Harvard School of Public Health, chairman of the Physicians Task Force on Hunger in America, said the administration had eliminated a program to inform people about food stamp benefits.

"For the president to lead the way in eliminating the federal outreach funding, and then turn around and say people don't know about the program, is correct but a little bit puzzling," Brown said.

The Children's Defense Fund, an advocacy group, reported in a study earlier this year that last year only 59 of every 100 persons in poverty received food stamps, compared with 68 of every 100 in 1980.

The study said the decline in food stamp coverage occurred because of eligibility restrictions imposed by the administration and Congress in 1981 and 1982, because many do not know they are eligible and because of a "lack of coordinated government efforts" to inform the poor about benefits.

The Harvard study had earlier estimated that a minimum of 20 million Americans go hungry at least two days a month.

A new study from the physicians group released this week concluded that "as hunger and poverty have increased, an adequate response to it has not. This is particularly true of the federal food-stamp program."

The study said participation in the food stamp program has declined while hunger and poverty have intensified and faulted government "barriers" to broader participation.

A White House official said Reagan's comments were sparked in part by a televised news report he saw Tuesday evening about the Harvard study. At the same time, the study was sharply criticized by Assistant Agriculture Secretary John Bode, who said the findings "are grossly inaccurate."

"We don't need to go back to the days of door-to-door campaigns to sign people up for food stamps," he said.

On other topics in a rambling 30-minute appearance before the students, Reagan also defended his former deputy chief of staff, Michael K. Deaver, against charges that Deaver -- who left the administration to become a lobbyist -- violated conflict-of-interest laws.

"I think it is well for us to note that he was the one who asked for an investigation" by an independent counsel, Reagan said, "which I think shows some, his confidence in innocence."

Reagan also said that, in the last five years, the Soviet Union has built 50 times as many bombers as the United States and its NATO allies. Aides cited a recent opinion article by former national security adviser Robert C. McFarlane as Reagan's source.

According to the Center for Defense Information, a private group, the United States and NATO have 11,400 combat aircraft, while the Soviet Union has 6,775.