Several terrorist groups in Europe are believed to be cooperating in planning attacks on NATO targets, defense contractors and western government facilities, according to U.S. officials.

The suspicion of leftist terrorist coordination is fueled by physical evidence, proclamations from several groups and unspecified intelligence reports, officials said.

Because there have been more than a dozen bombings and other types of terrorist actions in West Germany, Belgium, Spain and Portugal the past two months, U.S. and North Atlantic Treaty Organization installations have bolstered security, but no special precautions have been taken against a joint attack, John W. Campbell, spokesman for the State Department's Office for Combatting Terrorism, said.

"If there is cooperation between the terrorists it probably strengthens their combined capabilities," Campbell said. "But I don't think it changes our stance except as a matter of degree, and . . . we're already at a pretty high state of alert."

In a related development, there have been highly visible precautions taken in Brussels in the past week by Belgian authorities reportedly warned of impending attacks. Armored vehicles have been posted outside several embassies, including that of the United States, and there have been increased street patrols by paramilitary police armed with submachine guns.

The Belgian press has reported that the Belgian Embassy in Beirut had received documents indicating that the Moslem extremist group Islamic Jihad (Islamic Holy War) was planning attacks in Brussels to mark the fifth anniversary of the Iranian revolution this week. The Belgian interior minister told reporters Friday that the increased security was "in response to certain information which has reached us," Reuter news service reported.

Concern about unified attacks from European terrorists has been heightened by an announcement earlier this month from France's Direct Action terrorists and West Germany's Red Army Faction that they were merging in Western Europe to strike NATO targets. Also, the Fighting Communist Cells, believed responsible for a rash of bombing attacks in Belgium, issued a recent communique dedicating one attack to the Red Army Faction.

There is also intelligence evidence of links between Direct Action and Basque terrorists, as well as contact between Direct Action and the Fighting Communist Cells, which appears to be the smallest and newest of the leftist terrorist groups, according to Edward Djerejian, a State Department spokesman.

Campbell said "we have no reason to question" reports last week that explosives stolen last summer in Belgium were used by three groups -- Direct Action, the Red Army and the Fighting Communist Cells -- in separate attacks on facilities in Belgium, France and West Germany, including an unsuccessful strike Dec. 18 against a U.S.-NATO school in Oberammergau, West Germany.

Djerejian told reporters Friday that "some time ago we learned that terrorist groups in several Western European countries had decided to attack targets associated with NATO, the governments which are members of NATO and businesses which have defense contracts. We have been in close consultations with other governments to meet this threat."

Defense contractors, including American companies with European offices, have not been given "any blanket warning" but "they're all in pretty close contact" with U.S. embassies or the State Department, Campbell added.

Although not tied directly to the European terrorists, cooperation has been particularly strong between U.S. and Italian authorities after a foiled plot last month by Islamic fundamentalists to blow up the U.S. Embassy in Rome, a Defense Department official said.

In addition to their operational links, the leftist groups are believed to be allied by opposition to NATO and its deployment of Pershing II and cruise missiles, and a general distaste for "western imperialist institutions," Djerejian said.

The Defense official speculated that the recent outburst of violent attacks in part reflects a frustrated reaction to the decline of mass protests and civil disobedience once directed against NATO's missile deployment plans.