Does 'Bulgarian trail' run through Belgium to NATO?

For weeks now, the Bulgarians have had their hands full denying rather racy charges originating in the West. First, there were the reported confessions of Turkish gunman Mehmet Ali Agca - sentenced to life for trying to kill Pope John Paul II - saying he had escaped from jail in 1979 with the help of a Turkish terrorist allegedly employed by the Bulgarians.

Then came the astonishing allegations by the Italian defense minister, Lelio Lagorio, linking the Bulgarian secret service, long thought to be under the direct control of the Soviet Union's KGB, to the assassination attempt. Earlier, several Bulgarians were also linked to a major drug- and arms-smuggling ring operating in northern Italy.

The new catch phrase: la pista Bulgara (the Bulgarian trail).

Now come sweeping charges that the Bulgarians have begun stepping up their spying activities in Belgium - home to NATO headquarters and a key ally in the Western alliance.

A senior Belgian government official told The Christian Science Monitor recently that the charges - contained in a series of articles in the respected Belgian French-language daily Le Soir - were being taken seriously.

''It's something that until now was almost completely unknown,'' the official said, adding that the Justice Ministry's ''special services'' had begun examining the affair to ascertain the truth.

According to Jean-Paul Collette, the Le Soir reporter who claims to have investigated the matter with some care, the Bulgarian spies have been posing as truck drivers, stopping their ''disabled'' vehicles near airports, military bases, and seaports, photographing everything worth photographing and even taking depth-soundings at strategic waterways.

Their alleged aim: to gather information not easily obtained by Soviet spies (posing as diplomats) already operating in Belgium, or by the sophisticated Warsaw Pact spy satellites now circling the globe.

The Le Soir correspondent says the Belgian authorities have become deeply ''uneasy.''

With some reason. Not only is Belgium host to NATO and its European military command but also it would be of extreme strategic importance should war break out in Western Europe. It is also one of only five West European countries to have agreed to begin deploying new US nuclear missiles at the end of this year if the US-Soviet arms talks now under way in Geneva show no ''concrete'' results by then.

The Le Soir reporter points to the increasing number of Bulgaria-licensed trucks coming to Belgium, wildly out of proportion to the volume of trade between the two countries and despite an international agreement limiting the number of trips by Bulgarian trucks to and from Belgium to 300 (compared with 2, 200, for example, for Czechoslovakia and 1,800 for the Soviet Union). Moreover, the bulk of Bulgaria's exports to Belgium consist of nonprecious metals, mineral oil, and precious stones - goods usually shipped by sea.

According to the newspaper, one Bulgarian truck driver has been seen taking soundings at the canal running between Ghent (an inland port designated as a point of disembarkation for US troop reinforcements) and the sea. Another was observed spending three days to travel a distance of 75 miles.