Military commanders of the largest Nicaraguan rebel group said yesterday they would be willing to capture and attempt to hold territory within Nicaragua if that would convince Congress to provide them with more military aid.

If successful, such an action could lead to the establishment of an alternative government that the United States could formally recognize and aid openly.

President Reagan said in a recent interview that the current leftist Sandinista government of Nicaragua "is not a government" but "a faction of the revolution," and that Congress should approve further aid to the rebels who "want the original goals of the revolution instituted."

In a related development, the human rights group Americas Watch released a report charging that both the Nicaraguan government and the rebel groups have been guilty of "serious violations of the laws of war." Vice chairman Aryeh Neier said the United States bears "major responsibility" for the rebels' abuses because it has provided them with $80 million over the past three years.

Enrique Bermudez, chief of military operations of the Nicaraguan Democratic Force (FDN by its Spanish initials), said in response to questions at a news conference that "no official request" had been made that the rebel forces abandon their hit-and-run tactics in favor of holding ground.

He said such a request would have to come "from the ones who are going to give the funds" because it would result in an all-out military confrontation with the vastly more powerful Sandinista army.

"We have the capacity to do it," Bermudez said. "It would be costly for us, but we will do any sacrifice in order to get the funds."

Many members of Congress who refuse to support further covert aid to the rebels are still highly critical of the Sandinistas, and the Reagan administration has been hunting for some way to provide the rebels with open assistance.

But officials have shied away from the idea of recognizing a rival government within Nicaragua, concerned that the rebels, faced with defeat in a pitched battle with the Sandinistas, might then demand direct U.S. military intervention. They have also tried to promote unity among the various rebel factions with an eye to a possible joint government-in-exile, perhaps in Costa Rica, that could receive open U.S. aid. But that has foundered on the refusal of the rebel group led by Eden Pastora Gomez, the charismatic "Commander Zero," to join forces with Bermudez, a former member of the National Guard of ousted dictator Anastasio Somoza.

Bermudez and three regional military commanders of the FDN with him at the news conference said the 50 top leaders of their alliance include only six former National Guard lieutenants, five sergeants and one enlisted man. "In a force of 15,000, what is this?" Bermudez said.

They said refusal by Congress to grant Reagan's request for $14 million in covert aid would be viewed in Nicaragua "as a signal that the United States is withdrawing from Nicaragua."

The four also denied charges that rebel troops are guilty of serious human rights abuses, calling the charges "propaganda" by the Sandinistas. Bermudez said the FDN attracted 500 new recruits per month last year. "If we were committing abuses against the people, the people would not support us," he said.

Neier, however, said none of the Americas Watch information came from the Sandinista government.

The 97-page report, which Neier called "more illustrative than comprehensive," documented two Sandinista massacres -- one involving 14 to 17 Miskito Indians in 1982 and another that killed seven Indians in 1982 -- and noted that 70 Miskitos disappeared after their arrest in late 1982.

It said abuses by the Sandinistas have decreased, but that rebel abuses continue and involve "selective killing of civilians, indiscriminate attacks on civilians, mistreatment of prisoners . . . , hostage-taking and outrages against personal dignity such as rape."

The United States has "aided and abetted" these crimes by providing aid the report said.