International news agencies here today received letters purporting to come from terrorist groups in France and West Germany stating that they had united with the aim of attacking North Atlantic Treaty Organization targets.

French police said they were investigating the authenticity of the documents, which were written in the name of West Germany's Red Army Faction and France's Action Directe. The five-page letters said that the two groups were forming a joint "political-military front in Western Europe."

Attacks against military-related targets have taken place in West Germany, Belgium and France during the past year. Responsibility has been claimed by different groups, and until now, officials have said that they were unable to find evidence that the attacks were coordinated.

The Red Army Faction appears to have stepped up its activity recently after a period of retraction following a spate of terrorist attacks in the 1970s, including the hijacking of a Lufthansa airlines plane and the killing of business leaders. Action Directe has conducted a series of relatively minor bombings in France since 1980.

Copies of today's letters, which were written in both German and French, were received by the Paris offices of Reuter news agency and the French news agency, Agence France-Presse. They were mailed in Paris yesterday.

Agence France-Presse reported that West German police had succeeded in uncovering one possible link between the two organizations. Explosives used in a bomb attack against a NATO base at Oberammergau in West Germany last month were traced to the same stock of stolen explosives used in a bomb attack in Paris in August. Responsibility for the August attack was claimed by Action Directe.