PARIS, JAN. 22 -- France and West Germany, celebrating 25 years of postwar friendship, strengthened their ties today with formation of a high-level military commission to coordinate Franco-German policies on nuclear disarmament and other defense matters.

President Francois Mitterrand and Chancellor Helmut Kohl, at ceremonies here marking the anniversary of their two countries' friendship treaty, also announced creation of an economic commission to promote increased cooperation on trade and monetary policy.

The two leaders, with these and other steps, sought to give new impetus to an expanding defense and political relationship that has grown up between the former enemies since Charles de Gaulle and Konrad Adenauer signed the treaty Jan. 22, 1963, to signal a formal burial of the enmity engendered by World War II.

Although it is only part of the relationship, Franco-German defense cooperation has become particularly important in an atmosphere of uncertainty and change created in Europe by the Dec. 8 treaty abolishing U.S. and Soviet intermediate-range nuclear missiles stationed in Europe.

High-level French officials repeatedly have expressed fear that diminished U.S. nuclear protection in Europe could lead West Germany to look eastward for its security. Against that background, Mitterrand and Prime Minister Jacques Chirac have emphasized in recent months an increasingly explicit commitment to defend West German territory with French nuclear might.

In addition, they have sought to increase defense cooperation as much as possible, while recognizing that such cooperation is limited as long as West Germany is part of the NATO integrated command and France is not.

The French government responded enthusiastically, for example, to Kohl's suggestion last June for a Franco-German brigade to symbolize the two countries' growing defense ties. But West German troops for the brigade had to come only from the Territorial Army, or reserves, because regular German soldiers fall under the NATO command.

Officials announced today that the 3,000- to 4,000-soldier brigade's first commander will be a Frenchman, Brig. Gen. Jean Sengeisen. He will be replaced by a German in two years on a rotation basis, they said. The unit is to begin operations Oct. 1.

The Franco-German Defense and Security Council, which will meet at least twice a year, includes both countries' chiefs of state and government, defense ministers and military chiefs of staff, the officials said. Among other things, it will complete negotiations for setting up the joint brigade and work out "common concepts" on defense and security, Kohl said in a speech.

"We are well aware as we do this that in practice it will not always be simple to carry out this decision," Kohl acknowledged. "But we have to harness ourselves to these tasks if we want to move into concrete reality the goal defined by President Mitterrand as our community of destiny."

French and West German leaders have already differed sharply on what to do in the wake of the intermediate-range missile accord. West Germans have expressed interest in moving toward abolishing Europe's short-range nuclear weapons as well, while the French insist that significant strategic and conventional arms reductions must come first.

Similarly, French and German economic leaders often have been at odds during the recent monetary disruptions, with France joining other nations in calling on Kohl to take steps to expand the West German economy. For this reason, officials said it was significant that French and German central bank heads were included in the economic commission along with finance ministers.

Increased cooperation between Bonn and Paris, while applauded in Washington, has evoked expressions of concern in other European capitals.

Foreign Minister Guilio Andreotti of Italy, for example, warned recently that overemphasis on French-German cooperation could lead to diminished attention to and resources for the defense of southern Europe.

Prime Minister Margaret Thatcher of Britain, while refraining from direct criticism, called last week for Atlantic Alliance countries outside the NATO integrated command to increase "concretely" their cooperation with the organization's military planners.

Similarly, Foreign Minister Leo Tindemans of Belgium expressed hope in an interview published today that the increase in Franco-German cooperation will lead to "better coordination" between French forces and NATO.

"There is no Franco-German axis," Mitterrand said in an allusion to these concerns. "But there is a strong desire in Bonn and Paris to pool our resources to move forward with Europe."

In the same vein, Kohl said Franco-German defense cooperation should lead toward "a common European defense and a European army, including our European friends and in the framework of confident relations between partners with the United States."