Hoping to breathe a little life into the long-stalled U.S.-Japan airline negotiations, Ambassador Mike Mansfield yesterday said the United States is prepared to offer new concessions for Japanese passenger planes flying to and through American cities.

Mansfield said in a speech that Japan would be offered access to more American airports and new rights to fly on to other countries after first stopping in U.S. cities.

In exchange, he said, Japan should open up its airports to more charter flights and should agree to lower air fares, as the United States airlines are doing on European routes.

Mansfield's speech to a group of American businessmen was an obvious attempt to put some public pressure on Japanese government before the next round of negotiations expected to begin here in November.

It appeared to have little effect. A foreign ministry source welcomed the offers but said the U.S. wants to move too far too fast in liberalizing air fares and adding charter flights into this country.

The airline conflict is one of the toughiest issues existing between the two countries. Each regards the other government striving to protect its own airlines from foreign competition.

Japan regards the present 26-year-old agreement as unequal and wants an American acknowledgement to that effect. Mansfield said yesterday the two sides should stop arguing about whether the pact is unfair or not and get on with negotiations for a new one.

American sources said the gist of some U.S. concessions had been made known to the Japanese during the last round of talks in March but were never acknowledged by Japan's negotiators. Japan's airline industry repeatedly describes the U.S. position as inflexible.

It was understood that the U.S. will offer new gateway ports to Japan's lines in Seattle and Chicago and possibly in other cities, in addition to the seven presently served.

Also, the U.S. is said to be willing to permit Japanese planes to fly beyond its American stops to other countries. Japan is especially eager to get socalled "beyond-rights" to cities in South America.

In addition to lower transPacific air fares, the U.S. is asking Japan to vastly increased the number of charter flights permitted into this country. An American source said "several hundred" charter flights a year, to be divided between U.S. and Japanese air carriers, will be proposed.

The current agreement was reached in 1952 just as the U.S. occupation of Japan was ending. Japan has maintained that the agreement was unfair to begin with and has been steadily weighted even more against this country with each revision. "The USA wrote the rules and continues to call the tune," declares an angrily worded statement being circulated by Japan Air Lines, the national flag carrier.

The Japanese foreign ministry's reaction to Mansfield's speech yesterday did not seem to offer much hope that the stalemate will be broken when the negotiations resume in November.

A source there said his speech was "welcome" if it clearly recognized that the inequalities in the current agreement must be changed.

But he said that U.S. negotiators want to move "too fast" in lowering air fares and increasing charter flights into Japan. Those goals may be achieved gradually, he said, but no immediately.

He also restated Japan's argument that additional flights must be limited because of technical problems around major airports. The jet fuel supply is insufficient at Tokyo's new international airport and there are noise-pollution problems at Osaka's, he said.