Several key senators including important backers of the Panama Canal treaties are likely to demand a clarification of Panama's attitude toward a key amendment to the first canal treaty, Senate sources said yesterday.

This demand could pose a new and potentially serious Senate obstacle to the treaties.

The amendment in question is a "reservation" to the first treaty - already approved by the Senate - that asserts America's right to intervene militarily in Panama after the year 2000 to keep the canal open.

The government of Panama yesterday issued a formal statement denying that it has already rejected the reservation. This was a response to published reports that the reservation had so angered the Panamanians that they could not stomach it.

Despite Panamas denial, reliable sources said several key senators were worried and angered about messages Panama has sent to other governments disapproval. One of these messages, a letter signed by Panama's leader, Gen. Omar Torrijos, suggested that the reservation violated the charters of the United Nations and the Organization of American States.

News of these messates circulated in Washington for the first time Thursday. Yesterday key Senate aides said Panama's denial that it had already rejected the reservation - known as the DeConcini reservation for Sen. Dennis DeConcini, the Arizona Democrat who introduced it - would not allay doubts about Torrijos' ture intentions.

Some sources, including an aide to DeConcini, suggested that he or other senotors might demand that Tarrijos publicly embrace the DeConcini amendment before agreeing to vote for the second canal treaty, which will be up for a final vote on April 18.

U.S. diplomats and Panamanian officials have said it would be politically impossible for Torrijos to publicy endorse the reservation.

Washington Post special correspondent Marlise Simons reported from Panama City last night that government officials there insisted they had no intention of rejecting the treaty because of the DeConcini reservation.

Simons said one senior Panamanian official told her that the Torrijos government was split over the messages raising doubts about the reservation. That official personally opposed sending the messages, she reported.

Simons also reported that political tensions in Panama are high, and several opposition groups are preparing to pressure the government to reject the treaties because of the DeConcini reservation.

Panama's denial that it had rejected the DeConcini reservation was being circulated by Carter administration officials yesterday, and in interviews several said they thought the flap had quickly passed.

But Senate emotion seemed much stronger. One indication was a public statement Thursday night by Howard H. Baker Jr. (R-Tenn.), the minority leader and a key backer of the Panama treaties:

"I've really gone out on a limb for these treaties because I thought they were right for my country, and I still think that. But, I think that our friends in Panama ought to know that just a twitch of an eyelid, just the slightest provocation or expression that these treaties, or this treaty in this form, is not acceptable to Panama, and this whole thing could go down the tube."

Other senators who voted for the first treaty guaranteeing the future neutrality of the canal said yesterday that the Senate would not accept any dilution of the DeConcini reservation.

Frank Church (D-Idaho), floor manager for the treaties, said he had told Panama's ambassador in Washington that the Senate "would not consider any action designed to subvert the DeConcini amendment."

Sen. Sam Nunn (D-Ga.) said any attempt to undercut the reservation would, in his opinion, "kill the treaty."

Treaty opponents jumped on the Panamian messages raising doubts about the DeConcini reservation as proof of bad faith, Sen. Bob Dole (R-Kan.), for example, said the step showed that Panama would not abide by the treaties, so they should be rejected by the Senate.

One Senate aide noted that by inflaming senatorial opinion, Torrijos may have obligated himself to embrace the politically painful DeConcini reservation to assure Senate passage of the second treaty, whereas if he had said nothing, no public embrace would have been demanded.

However, correspondent Simons reported from Panama City that it would be extremely difficult and perhaps politically impossible for Torrijos to publicy endorse the DeConcini reservation now.