Greece announced today that it has decided to buy 40 U.S.-made F16G combat aircraft and an equal number of French Mirage 2000s in a reported $2 billion purchase intended to update its Air Force, currently a mix of 20 older types of planes.

The inclusion of the French plane in the package was seen by political observers as a reflection of the Socialist government's commitment to diversifying Greece's sources of arms away from the United States. The F16 was said by military observers to have been favored by the Air Force, which is more familiar with U.S. equipment than with European.

No official price tag was given by government spokesman Dimitris Maroudas, who said the decision for the mixed purchase was made after an Economics Ministry recommendation to Prime Minister Andreas Papandreou, who also serves as defense minister. But military experts estimated the cost at more than $2 billion, making this the largest weapons purchase in Greek history. Maroudas said details remain to be worked out before final signing of the contracts.

The new planes will replace squadrons of F104 bombers and F5 reconnaissance aircraft. General Dynamics, manufacturer of the F16, and Dassault, maker of the Mirage, won out against fierce competition from Northrop and McDonnell Douglas and the British-West German-Italian consortium Panavia.

The decision balances political, military and economic interests. Buying the Mirage 2000 will allow the Socialists to claim that they are fulfilling promises to reduce Greece's military dependence on Washington. But the F16 was included to keep Greece's air capability on a par with rival Turkey, which agreed last year to buy 160 F16s. The two countries, whose relations are strained over a variety of issues, are in a fierce race for U.S. military aid and equipment.

Also, Greece will be able to use the $500 million in annual foreign military sales credits it has received from Washington in exchange for the operation of the four U.S. military bases here.

The Greek government has been wrestling with the planes decision since coming to power in October 1981, when it announced that it planned a mammoth purchase of 100 to 130 fighter jets to modernize its Air Force. The number was reduced to 80 as costs rose.

Sales packages submitted by rival manufacturers during the bidding included offers for the manufacture of parts and assembly by Greece's state-run Hellenic Aerospace Industry, which hoped to use the deal to expand its activities from repair and maintenance to manufacture and assembly. Today's announcement, however, did not say what role, if any, the Greek firm would have in the purchases.