National security adviser Colin L. Powell said yesterday that Nicaragua's plans to import large amounts of advanced Soviet weaponry and double its armed forces to 600,000 men pose a "direct threat" to the nation's Central American neighbors.

He said the Reagan administration will "follow up" on remarks made by General Secretary Mikhail Gorbachev this week to see if Soviet weapons shipments can be curtailed.

Powell's comments came amid criticism by members of Congress and others of Nicaragua's newly revealed plan to have one-fifth of its population under arms by 1995 and to equip them with Soviet-made planes, missiles and artillery.

The United States has warned the Nicaraguan Sandinista government against acquiring MiGs, and Powell repeated yesterday that such action would be viewed with the "greatest seriousness" and "would not be in the interest of U.S.-Soviet relations."

Powell said that although President Reagan and Gorbachev had made "no deal . . . the general secretary made a few cryptic references to an arrangement where they would reduce their level of arms support to something in the neighborhood of small arms, or he even said police weapons at one time."

"And he made reference to our doing likewise. We'll follow up on that to see what it really means, but you have to put it in context with what was in The Washington Post this morning," Powell said.

He was referring to a report on a speech by Nicaraguan Defense Minister Humberto Ortega detailing plans for the 15-year military buildup.

Vice President Bush, appearing on NBC News' "Meet the Press," said Gorbachev "did discuss not sending major weapons. In fact, he talked about 'police weapons' . . . . I . . . was quite surprised, frankly."

"Look, it is my view that we should continue to help the contras until we have the Sandinistas agree with what they told the Organization of American States they'd do," Bush said. "Namely, go down democracy's road with free elections, freedom of worship, freedom of the press. And so we have to keep the pressure on."

In another television interview, White House chief of staff Howard H. Baker Jr. called Gorbachev's comments about Nicaragua "intriguing."

The exchange between Reagan and Gorbachev occurred during their 11-minute private walk around the White House grounds. "It only lasted about 15 seconds and I don't know what he meant," Baker said.

Baker paraphrased Gorbachev as saying, "Maybe in Central America, maybe in Nicaragua, we could just stop sending any arms except small arms for police work to the Sandinista government if you will stop sending assistance to the freedom fighters, to the contras." Baker appeared on the Cable News Network program "Newsmaker Sunday."

Gennadi Gerasimov, the Soviet Foreign Ministry spokesman, ducked persistent questioning on the Brinkley program about MiG sales. "If and when this question is going to arise, it will be decided within the governments. It is not yet there. It is not on the agenda."

Maj. Roger Miranda Bengoechea, a former top aide to Humberto Ortega who defected to the United States on Oct. 25, told reporters in an interview published yesterday that the Sandinistas were insecure about their relationship with the Soviets and "afraid that the Soviets will negotiate over their heads."

Miranda said that Nicaragua's rulers see their request for a squadron of MiG21s, which the Soviets have promised but never delivered, as a test of the Soviets' true intentions.

"The MiGs are of no use for guerrilla warfare," Miranda said. "But because the United States says they will not permit them, if the Soviet government permits them, it means the Soviets are saying to the United States, we are taking responsibility for the Sandinistas. What they've wanted is an umbrella of protection. Sometimes the Soviets have wobbled a little bit on that."

On a related matter, Gerasimov said Reagan's Strategic Defense Initiative is "on the back burner."

"We think it can ruin strategic stability if it is being put into effect," Gerasimov said. "But this is for the future. So this is why I'm saying it's on the back burner."