President Reagan said yesterday that the Nicaraguan contras "have to be sustained" because the pending Central American peace plan is insufficient to force the leftist Sandinista government to restore democracy in Nicaragua.

Reagan said in an interview with U.S. News & World Report that he had told Pope John Paul II Friday in Miami "that there are loopholes in that treaty that we're positive the Sandinistas would take advantage of."

And, in his weekly radio message, delivered from Camp David, the president sharply questioned the practicality of the peace plan and the motives of the Sandinistas. Reagan said the Central American plan "falls short of the safeguards" in a plan he submitted last month with House Speaker Jim Wright (D-Tex.).

The Reagan-Wright proposal was rejected by five Central American nations, including Nicaragua, which instead accepted a proposal made by Costa Rican President Oscar Arias.

Secretary of State George P. Shultz announced Thursday that the administration intends to seek a new $270 million aid package for the contras, a tactic that was denounced by Wright and other Democratic leaders as undermining the regional peace attempt. Reagan said in the interview that the contras have to be sustained until an agreement has been put in place that brings "complete democratization" to Nicaragua.

In his radio speech, the president castigated the Sandinistas and their Soviet supporters.

"Many Americans have learned over the last few months what has really been happening in Nicaragua, how a democratic revolution was betrayed, how a tiny elite has been creating a totalitarian Marxist-Leninist dictatorship to satisfy their own personal lust for power and to give the Soviet Union a beachhead on the mainland of this continent, only 2,000 miles from the Texas border -- a clear national security threat," Reagan said.

But in the interview, Reagan reiterated the administration view that the Nicarguan situation should not be linked to a pending U.S.-Soviet agreement that would remove medium-range nuclear weapons from the arsenals of both superpowers. Reagan said he would like to go beyond that treaty and make progress on another long-discussed accord that would reduce intercontinental nuclear ballistic missiles, but he did not give specifics.

Reagan appeared to take contradictory views on the 1972 Antiballistic Missile (ABM) Treaty, which the administration has sought to reinterpret to allow deployment of a missile defense system.

At one point Reagan said that, because of Soviet violations, "we should look realistically" at the treaty to see if it should be retained. But when the president was asked if he wanted to "scrap the treaty," he responded, "Well, no, I wouldn't favor that right now . . . for a practical reason. I think that they {the Soviets} are much more prepared to take advantage of such a thing than we are."

Reagan also was asked whether he would be willing to sit down with Iranian ruler Ayatollah Ruhollah Khomeini if the Iran-Iraq war ended.

"I think you better ask him the question, would he sit down with me?" Reagan replied. "You know, I'm the 'great Satan' and he might be disappointed that I didn't show horns when I got there. But, no, I'm willing to talk with anyone on things of that kind where it's legitimate and between governments."

Reagan, who has been trying to avoid questions about the Iran-contra scandal, then launched into an unsolicited defense of the affair, which he has acknowledged became an attempted trade of U.S. arms for American hostages being held in Lebanon.

The president said that "misconceptions" still exist about his intentions and repeated a previous assertion that "we were not doing business with the ayatollah in that covert operation . . . . We were doing business with individuals who wanted to discuss a relationship with the government that might follow the ayatollah."