BONN, JAN. 18 -- France and West Germany could create joint naval units as a follow-up to plans to establish a joint army brigade, French President Francois Mitterrand said in an interview published today.

The West German government would be happy to consider the idea, although it is not yet being formally discussed, a West German official said.

Mitterrand's comment, in an interview with the West German newspaper Die Welt, came as Italy joined Britain in expressing reservations about increasing Franco-German defense cooperation.

In a separate development, West German officials said that the North Atlantic Treaty Organization was likely to hold a summit in Brussels on March 2-3.

President Reagan and other NATO leaders are expected to hail the intermediate nuclear forces treaty and to discuss disarmament and defense issues, West German and NATO officials said.

France and West Germany plan to announce details of a 3,000-man joint brigade, and the formation of a Franco-German defense council, in Paris on Jan. 22.

In the interview, Mitterrand said, "Since there will be joint army units, I don't see why there shouldn't be joint naval units as well."

The United States has welcomed the increased Franco-German cooperation as a way of strengthening Western Europe's self-reliance in defense and maximizing France's military links to the alliance.

But Italian Foreign Minister Giulio Andreotti, echoing comments made late last year by British Prime Minister Margaret Thatcher, expressed concern that formation of a "Franco-German axis" was not healthy for the allies.

Andreotti, in an interview published in yesterday's edition of the Rome daily La Repubblica, said such developments risk "encouraging certain tendencies in America, which are owing partly to budget reasons, favoring a diminishing of their presence in Europe."