THe government of Panama has circulated a statement to members of the United Nations raising the possibility that it will challenge or even reject a key amendment to the first Panama Canal treaty approved last month by the Senate.

This unexpected development raises new uncertainty about the fate of the Panama Canal treaties just as their supporters in the Senate have become increasingly confident of winning two-thirds approval for the second treaty now being debated.

It was already clear that Panama disliked the disputed "DeConcini reservation" to the first treaty, spelling out an American right to intervene in Panama to keep the canal operating at any time after the year 2000, when Panama would assume full control over the canal. The reservations was introduced by Sen. Dennis DeConcini (D-Ariz.).

But the letter Panama has circulated at the United Nations - and, reportedly, a second letter sent by Panama's leader to certain heads of state - goes beyond mere dislike; and suggests the Panama may find the reservation unacceptable. If that were happen, the treaty could not be properly ratified, and the long effort to resolve the future of the canal by negotiations could reach an impasse.

The Carter administration is worried by this turn of events, officials said yesterday, but not yet alarmed.Several official sources said that Gen. Omar Torrijos, the Panamanian leader, has not yet made his true intentions clear.

Some theorized that Torrijos is trying to pressure the Senate not to make any more significant changes in the treaties. Others suggested Torrijos might be responding to domestic opponents who have attacked the DeConcini reservation. Yet another theory advanced by officials was that Torrijos was testing the waters of world opinion to see what sort of wupport he might have if he tried to reject the reservation.

Columnist Carl T. Rowan reported on WTOP-TV Wednesday night that Panama had already told the United States it could not accept the DeConcini reservation, but administration officials heatedly denied this yesterday. No evidence could be found to support Rowan's interpretation.

Administration officials said that Panama has told them more than once how much it dislikes the DeConcini reservation. Panamanian and U.S. officials have discussed ways to "put a good face on it," one official said.

Yesterday, Panama's ambassadors to the United States, the United Nations and the Organization of American States paid a call on Sens. Alan Craneton (D-Calif.) and Frank Church (D-Idabe), two key treaty supporters, to express concern about the situation in Panama as a result of the DeConcini reservation.

Demonstrations are scheduled for today in Panama City to protest the reservatiom.

Ambassador William Jorden in Panama has cabled reports to Washington indicating that the situation there is "very bad," according to one source who reads those cables.

Several Latin American government are known to be upset by the DeConcini reservation. President Carlos Andres Perez of Venezuela told President Carter personally in Caracas last week that he hoped the president could do something to mitigate the reservation's impact.

There have been discussions among Latin American diplomats about the possibility of some hemisphere governments refusing to ratify the treaty on the canal's neutrality, which will be open to adherence by all states, as a form of protest against the reservation.

The DeConcini reservation was accepted by Carter during tense negotiations for the last few votes nedded to win two-thirds approval of the first Panama treaty in the Senate. (The first treaty concerns preservation of the canal's neutrality; the second, now under consideration, spells out conditions for U.S. transfer of the canal to Panama between now and 2000.)

A number of liberal Democratic senators who support the treaties argued strongly against the DeConcini reservation as unwarranted and provacative, but administration officials decided they had to accept it to win over DeConcini and perhaps several other senators. (At least one other, Sen. Paul Hatfield (D-Mont.), said his "aye" vote depended on the reservation, and the first treaty only carried with one vote to spare.)

In the message circulated to U.N. members, Panama quoted one of those liberal senators, Edward M. Kennedy (D-Mass.:, who said the reservation "has the ring of military interventionism - not just during this century, but for all time."

The message also said:

"According to its proponent, the 'DeConcini Amendment' is intended to give to the U.S. the unilateral and perpetual right to 'take military action on Panamanian soil without the consent of the Panamanian government,' pretended that said amendment must be constructed to permit the U.S. to intervene in Panama in the event of labor unrest, strikes, a slow-down, or under any other pretext labeled as interference with canal operations."

The Panamanians attached to their message the text of Kennedy's criticisms of the reservation as well as DeConcini's claims for the United States. The message said Torrijos "considered it his ruty" to circulate this material to other countries.

When the DeConcini reservation was adopted last month, Panama said it would make no comment on it until the Senate completed action on the second treaty. This message amouted to a clear comment, however.

U.S. officials said Torrijos has apparently also ssent a separate letter to heads of state of certain governments repeating the gist of the U.N. message and adding that the DeConcini reservation violates the U.S. and OAS charters.

However, the United States has been unable to acquire a copy of this letter, officials said, and this report of its contents could not be confirmed.