The Carter administration intends to offer Nationalist China a new F5 fighter more lethal than present-day versions but not long-ranged enough to threaten Mainland China, officials said yesterday.

An internal Pentagon memo concedes that the proposed F5 sale may draw protests from those who contend it would violate President Carter's guildelines against developing weapons expressly for a foreign government rather than limiting sales to arms already in the U.S. arsenal.

But the administration is expected to argue that this Northrop F5G for Taiwan is really only an update of the existing fighter that has been sold around the world - an argument that is likely to be challenged in Congress.

Other protests are already being sounded by Northrop's competitiors in the aerospace industry, which contend the sale of new fighter planes to Nationalist China - which is expected to buy 60 of them - should come after an open competition rather than through a deal engineered by the government with one company.

The leading competitors to the F5G are the McDonnell-Douglas F4 and the General Dynamics F16. They apparently were rejected by the Carter administration because of the offensive threat they posed to Mainland China. Both have long range and carry a wide array of armament.

The Northrop F5G is being described by backers of the sale to Taiwan as halfway between the present day F5 and the F16.

"There's general agreement on this one," said one administration official yesterday in stating that leaders at State, the Pentagon and White House have recently agreed that the Northrop F5G offers the best hope of satisfying Taiwan without alarming Peking.

President Carter, sources said yesterday, has not formally approved the proposed F5G sale to Taiwan.

The president, in issuing his guidelines on May 19, 1977, for selling U.S. arms to other nations, said, "development or significant modification of advanced weapons systems solely for export will not be permitted."

The F5G on Northrop drawing boards, which could be ready by 1984, has one big engine rather than the two smaller ones in the F5s now flying. The extra power and other advantages stemming from the big engine would enable the F5G to carry the U.S. Air Force Sparrow missiles as well as Sidewinder missiles now used on F5Es. Both the Sparrow and Sidewinder are missiles to destroy other airplanes.

With this emphasis on air-to-air combat rather than on bombing, the F5G can be protrayed by the Carter administration as a defensive weapon rather than offensive weapon of concern to Mainland China.

Less clear is whether the F5G would represent merely an update of the existing F5E fighter or a development or significant modification" of an "advanced weapons system solely for export," which Carter has promised to prohibit.

The Pentagon went to Northrop to request a new version of its F5 specifically for Natioalist China, according to an Aug. 21 article in Aviation Week & Space Technology magazine. Maj. Gen. Timothy I. Ahern, U.S. Air Force assistant deput chief for research, told the magazine that the Pentagon had asked his command to explore ways to arm the F5E with the large Sparrow missile.

The F5G apparently grew out of this Pentagon initiative.

Taiwan's first choice up to now has been the long-range F4 fighter, which the administration does not want Taiwan to have.

The Taiwan government, which has received oil at cut rates from Saudi Arabia and thus would be sensitive to its views, turned down Carter's offer in July to buy the Israeli-made Kfir, which is somewhere between the F5 and F16 in sophistication.

It is not known how much money would be involved in the F5G sale to Taiwan. The Taiwan government currently produces its own F5Es under license. The sale of the new fighter would put additional pressure on Carter's effort to hold down foreign arms sales.