Nicaragua has agreed to discuss with the United States the Sandinista government's ties to leftist guerrillas in El Salvador and other key points of difference between the two countries.

The agreement came in a detailed response to an eight-point proposal for negotiations put forth by the Reagan administration in April.

The Reagan administration's repeated charges that Nicaragua is transporting arms to Salvadoran rebels, repeatedly denied by Nicaragua, have been the key stumbling block in attempts to begin serious talks between the two governments.

In a diplomatic note submitted yesterday to the State Department by Nicaraguan Ambassador Francisco Fiallos, the Nicaraguans did not concede that they have materially aided the Salvadoran rebels. But they side-stepped the question in careful language rather than issuing a flat denial and called once again for flexibility from both sides in negotiations that would begin "with no previous conditions regarding content."

Sources familiar with Washington's most recent note to Nicaragua on the negotiations issue said it offered the possibility that Nicaragua could end its alleged involvement in arms trafficking to El Salvador without making any public admission.

Nicaragua's only flatly stated prerequisite for talks was that Mexico, which began a diplomatic initiative to relieve the growing tension in Central America in February, should be a "witness" and "host" for the negotiations. The United States has expressed a preference for bilateral talks, but it has not publicly ruled out some kind of role for Mexico in setting up the negotiations.

A U.S. Embassy spokesman said there would be no immediate comment on the Nicaraguan note.

The new Nicaraguan initiative follows expanded accusations by the United States in the informal diplomatic correspondence between Washington and Managua directed toward starting negotiations.

The Nicaraguans respond in sharp language to charges in an American note a week ago that they are not only aiding the Salvadoran insurgents but also destabilizing Costa Rica and Honduras as well.

The Nicaraguans term this "outrageous" and maintain that if anybody is being destabilized it is Nicaragua by the United States. They conclude that "in unilaterally and arbitrarily extending the list of accusations you the United States only make it more difficult to believe that your government is serious in intending to negotiate effectively."

Some U.S. officials have expressed suspicions that Nicaragua's government is more interested in propaganda than the substance of negotiations.

The latest note, which was not publicized, does contain some sarcastic language, but the essential message it conveys repeatedly is that nothing should be ruled out as a subject of discussion.

The note sought, as did the first response to the Reagan initiative on April 14, to raise the issues of U.S. spy flights over Nicaraguan territory, U.S. warships off its coast, reported covert operations against the government and the possibility of Washington backing hostile actions against Nicaragua by other groups.

With Nicaragua's weakening economy on the verge of disaster, the Sandinistas also want Washington to stop working against funding from multilateral lending institutions and friendly Western countries.

These five issues are raised in the context of Washington's third negotiating point calling for "a joint declaration between our two governments on the desirability of friendly relations," a declaration the Nicaraguans say would be hard to make at the moment considering overt and covert U.S. hostility.

In responses to the other points raised by the Reagan administration, the Nicaraguans said:

* The Sandinistas referred to American insistence on observation of human rights, political pluralism, freedom of the press and elections here as an "inexcusable position of interference" with Nicaragua's sovereign right to set its own domestic policies, but they also pointed to its "sovereign decision" to hold elections in 1985.

* Administration statements against "counterrevolutionary" training camps in the United States are "insufficient" when Nicaragua wants the dismantling of the camps. The note adds, however, that "at present the offer is of little relevance since the bands trained in these camps have for the most part been transferred to Central America and put into operation against our revolution."

* Nicaragua "would be willing to exchange viewpoints and discuss the issue" of limiting arms and military forces in Central America with the countries of the region but does not feel the United States is "the appropriate interlocutor" since Nicaragua's arms buildup is not a direct threat to the security of the United States.

* International verification mechanisms for compliance with any proposed arms reductions should come later and, again, without U.S. involvement.

* The Nicaraguans dismissed the U.S. suggestion of including a resumption of U.S. aid in the negotiations, saying, "Under no condition do we want the discussion of serious problems of a political character to be affected by suggestions of economic cooperation, which might imply that political interests can be negotiated in terms of given amounts of dollars."

* The Nicaraguans questioned why the matter of cultural exchanges, which are already common on unofficial levels, should be brought up at all in the context of the negotiations.