In a demonstration of heightened Western concern about a possible Soviet invasion of Poland, the United States is sending four radar warning aircraft to monitor Soviet troop movements in Eastern Europe.

Gen. Bernard Rogers, supreme allied commander in Europe, was reported by Western military sources to have asked that the reconnaissance planes and two airborne tankers be sent by the United States to Ramstein Air Base in West Germany. The request came as defense ministers of the North Atlantic Treaty Organization (NATO) gathered in Brussels.

Pentagon officials told United Press International that the sending of the planes was "directly related" to the Polish situation, although the request was couched in the form of preparations for a regular air defense training exercise. The planes, known in military circles as AWACS (Airborne Warning and Control Systems), will themselves only be able to "see" across the border into East Germany, not all the way to Poland.

The step was the latest in a series of limited defense measures Western authorities tightrope walk between warning the Soviets off an invasion and, provoking one.

Another such measure was disclosed here today. The NATO command has canceled plans to disband the Alliance's standing naval force for the Christmas holidays -- plans that were to have taken effect last Wednesday -- and instead will keep the fleet of mixed-nationally ships in service, though at a low state of readiness. This was done, according to NATO officials, largely as a symbolic gesture to reflect the increased Western attention being paid to Poland.

A senior Western defense commander today effectively ruled out any direct military response should the Soviets move into Poland and cautioned Western defense forces to do nothing now that might add to mounting tensions in the European arena.

Following a briefing to NATO defense ministers, Adm. Robert H. Falls told reports that they had recommended that Western governments do little more to prepare for the possibility of a Polish invasion than to stay alert. Falls, a Canadian who currently chairs NATO's military committee -- the Alliance's highest military authority -- confirmed reports that Soviet troops have completed preparations for marching into Poland.

"The ability of the Soviets to intervene is such that they could do it with little warning indeed," the admiral said. "They are in a state of readiness now so that they could move, and we would hear about it almost after the event."

Falls added, however, that this did not mean the Soviets had made the political decision to invade. Moreover, he noted several options open to Moscow in staging an intervention, including the possibility of using Polish troops and seeking an invitation from the Warsaw government as a pretext, which would likely affect the timing and speed of the event.

Falls said he knew of no change in Soviet military activity around Poland during the last 48 hours since Washington officials disclosed the Soviets were ready to invade Poland. Asked about the state of readiness of East German and Polish troops, the admiral said neither force had as yet shown any "out of garrison" activity.

Questioned as to how NATO would respond to a Soviet invasion, Falls said this would be "fundamentally a political decision."

"The NATO alliance is a defensive alliance which does not include Poland," Falls went on. "And so it is terribly important that whatever the alliance does it does not exacerbate the situation and adversely impact on the intentions of the Soviet Union.

"But having said that, I think it would also be prudent that when the Soviets have demonstrated such capabilities, that we don't just go on as a sort of business as usual. I think it would be unwise to pretend nothing is going on, and that we do nothing to show our concern."

In Washington, Pentagon sources said one AWACS plane was already on its way to Ramstein in West Germany and would arrive there Wednesday. They said two were being sent from Tinker Air Force base in Oklahoma and two from Iceland. In addition to two KC135 aircraft needed for refueling, six C141 transports carrying support equipment are also involved in the operation, they said.

[The AWACS aircraft has a 200-nautical-mile radar range, allowing it to monitor all troops activities in East Germany and a large of Czechoslovakia but not those under way in Poland itself, the sources said.]