The Defense Department, after a period of relative inactivity in Central America, plans to conduct military exercises in the region later this year and early next year, including one maneuver that will send U.S. forces into El Salvador, knowledgeable officials said yesterday.

The Pentagon has stepped up its intelligence-gathering activities in the region, officials said. U.S. Army reconnaissance planes that track Salvadoran rebels, it was learned yesterday, send information through Panama and the United States to U.S. advisers in El Salvador, where Salvadoran officials "listen in" to the message traffic without being given information directly -- in an attempt to avoid violations of the War Powers Act.

Several hundred U.S. troops are scheduled to participate in early December with Honduran and Salvadoran soldiers in a maneuver called King's Guard along the Honduran-Salvadoran border, officials said. The plan calls for some of the activities to take place in El Salvador, in what would be the first time U.S. maneuvers in Honduras have spilled into that neighboring country, they said.

Gen. Paul Gorman, chief of the U.S. Southern Command in Panama, has proposed that several thousand troops participate in a months-long exercise called Big Pine 3 beginning early next year, perhaps in March. The Joint Chiefs of Staff has endorsed the plan, one official said.

The Reagan administration has played down military activities in Central America during the election campaign, scaling back its exercises and neglecting to announce some of the smaller maneuvers that take place.

After the election, however, Gorman and other Pentagon leaders are expected to push for a more active and visible U.S. military role in the region.

The aim of that presence is to help the Salvadoran government win its civil war against leftist guerrillas and to help CIA-backed "contra" rebels pressure the Sandinista government of Nicaragua, officials said. To accomplish both missions, the administration is likely to resume its emphasis on creating a military infrastructure in Honduras, which lies between El Salvador and Nicaragua, they said.

In addition, a series of small-scale exercises under the umbrella name Bigger Focus '84 is to continue through February, perhaps expanding from the platoon-sized maneuvers that have been taking place this fall. The Pentagon has not announced most of those small exercises, three of which are taking place now.

A Marine contingent that left its radar and listening post in the strategically located Gulf of Fonseca last spring returned to Tiger Island late in August, again without public announcement. The Marines operate electronic listening equipment but did not reinstall their radar, officials said.

In addition, U.S. planes have increased their reconnaissance flights over El Salvador and other countries in the region, officials said. The CIA plane that crashed into a Salvadoran mountain last week, killing four CIA employes on board, was one indication of that activity.

An Army intelligence unit operating out of Honduras also has been flying reconnaissance missions over El Salvador, tracking guerrilla positions to help the Salvadoran army in its civil war.

In an attempt to avoid violating the War Powers Act -- which would prohibit "coordination" between the Army units and the Salvadorans -- information gathered by the planes is sent through U.S. facilities in Panama to listening posts in the United States and then to U.S. advisers in El Salvador.

Salvadoran officials know which frequency to listen to as messages reach the advisers an hour or so after the information is gathered, thereby avoiding a direct connection, officials said.

In the past year, the U.S. military in Honduras has built or improved eight airstrips, four base camps and two radar stations, one of which continues to provide information around the clock. About 1,000 U.S. military personnel are in Honduras now, according to Pentagon officials.

In addition, three small exercises -- involving platoons of 30 men or so -- are taking place, according to Pentagon spokesman Lt. Col Richard Lake. They include an engineering exercise, in which wells are being dug to support future military exercises in remote areas; a medical exercise and a counterinsurgency exercise involving Army Rangers.

Plans for Big Pine 3 have been complicated somewhat by friction with the Hondurans, who recently have pressed for more U.S. aid in exchange for their military cooperation.

One official said that Gorman had hoped to begin Big Pine 3 in December but that the date slipped because of ongoing negotiations with Honduran military leaders.

Honduras also has indicated that it does not want U.S. Green Berets to continue training Salvadoran roops on Honduran soil. As a result, the Southern Command is helping El Salvador prepare to do more training at La Union in El Salvador.

Toward that goal, the U.S. Army began training about 125 Salvadorans at Fort Benning, Ga., about a week ago, knowledgeable officials said. Those Salvadorans in turn will become trainers in their country, where the administration has promised not to deploy more than 55 U.S. trainers.

Officials said the King's Guard exercise may be used to get U.S. troops, who otherwise would be prohibited under the 55-man limit, into the La Union area to help in the training. Pentagon spokesmen declined to discuss King's Guard, however, and it could not be learned how long the maneuver will last or how many U.S. troops will be involved.