A secret briefing paper prepared for a meeting of U.S. ambassadors in Panama this week says that U.S.-backed forces in Nicaragua and El Salvador do not appear near victory and that the Reagan administration will seek additional aid for the region.

The paper said the administration will seek a supplemental appropriation from Congress to bring the aid "to the levels recommended" by the Kissinger commission on Central America. A congressional source said that amounts to more than $480 million.

Entitled "The View From Washington," the Sept. 4 draft paper also expresses administration doubts about peace negotiations in Central America and suggests that in his talks there, Assistant Secretary of State Elliott Abrams make these points:

*The dialogue between the government of El Salvador President Jose Napoleon Duarte and rebel forces "has been useful. We now need to consider whether and how to foster it. It does not appear that the war can be won by either side on the ground."

*"Nicaragua will remain our major problem and the chances for a breakthrough there are limited . . . . The armed Nicaragua resistance is a potent force but still a long way from success. Public support is growing but is still tenuous. They must understand the absolute necessity of fighting a 'clean' war."

*The United States prefers to continue the Contadora peace process -- sponsored by Mexico, Colombia, Venezuela and Panama -- but "collapse would be better than a bad agreement." The role and influence of a "support group" composed of Argentina, Brazil, Peru and Uruguay "is yet to be determined. It may breathe new life into the process.

"We are afraid that it will seek to pressure the U.S. and friends to accept an agreement rather than a good agreement . . . . We need to develop an active diplomacy now to head off efforts at Latin solidarity aimed against the U.S. and our allies, whether they are sponsored by the support group, the Cubans or the Nicaraguans. We need to find a way to turn pressure they bring to bear on us or our friends to our advantage."

*"We intend to seek a FY 1986 supplemental appropriation to increase assistance to the levels recommended by the Kissinger commission report. Congressional support is not assured." A congressional source said such an increase would mean $300 million in additional economic aid for the region as well as $182 million more in military aid. Such a request would be very controversial, the source added.

Another congressional source said, "Someone's feeding at the silly trough down there" if they think such an amount would be approved by Congress.

The paper also said, "The economic and related crises in South America and the Caribbean are serious and, given limited U.S. resources, difficult to address. We are working on them on a case-by-case basis, seeking to be catalysts for change, not agents of change."

It emphasized that the region's governments "must understand, with no ambiguity, that the U.S. is dedicated to the process of democracy and that the process involves more than just periodic elections. Respect for human rights and a functioning judiciary are essential."

A State Department spokesman said the draft briefing paper "was intended to provoke internal discussion" but "indicates how much our private policy papers compare to what we say publicly."

U.S. ambassadors from Nicaragua, Honduras, El Salvador, Guatemala, Costa Rica, Panama and Belize are scheduled to join Abrams and representatives of the National Security Council, the Central Intelligence Agency and the Defense Department at the session that begins today.

Topics will include the Reagan administration's plan for implementing $27 million in nonmilitary aid approved by Congress for counterrevolutionaries, called contras, fighting the government of Nicaragua, the administration's proposal for a regional counterterrorism program and a "current regional threat briefing" by Gen. John R. Gavin, head of the U.S. Southern Command.

Government officials in Honduras announced last month that they would oppose attempts to filter new aid to the contras through the U.S. embassy in Tegucigalpa. The briefing paper said, "We have to reassure our allies of our commitment to respect their domestic political constraints and must work closely with them regarding the details of the distribution of this assistance. Honduran support for the Nicaraguan resistance is critical to our efforts."

On other topics, the briefing paper said, "The prospects for additional transitions to democracy in the short-term are virtually nonexistent."