Mexico told the U.N. Security Council yesterday that the United States and Nicaragua had agreed to meet next month in Mexico City at a "high political level" to discuss their differences, but the State Department called the announcement premature.

"No such meeting has been agreed to," said U.S. spokesman Dean Fischer. "We have in the past made clear that we are willing to address the issues with Nicaragua . . . at the appropriate time."

The United States previously has singled out Mexico's initiative for peace in Central America as a positive contribution and authorized Mexican Foreign Minister Jorge Castaneda to act as an intermediary. No immediate explanation was offered for yesterday's apparent setback in this cooperation. Mexico has called for talks between the United States and Nicaragua as well as Cuba and has promised to facilitate similar dialogues between Nicaragua and its Central American neighbors.

This week, the State Department welcomed as "worth exploring" yet another regional peace plan, presented by Honduras Monday at a meeting of the Organization of American States. The proposal calls for internationally supervised arms limitation and a cutback in foreign military advisers in the region.

Honduran Foreign Minister Edgardo Paz Barnica discussed the plan Thursday with President Reagan and with Secretary of State Alexander M. Haig Jr., whom Paz said were enthusiastic. He said the foreign ministers of El Salvador and Costa Rica, who also attended the meetings, supported the plan.

Honduras recently became the only Central American country besides Costa Rica to democratically choose its government. The poorest of the region's five countries, it is trying to fend off the guerrilla warfare engulfing El Salvador and Guatemala. Nicaragua and Honduras exchanged charges this week of attacks on ships and planes.

The Honduran plan calls for a regional "reduction of arms and military forces strictly to the levels necessary for defense . . . and maintenance of public order." It also would seek accord on "reduction of foreign advisers, military and otherwise."

The wording seems aimed at a professed concern of the United States, the allegedly increasing numbers of Cuban advisers in Nicaragua. However, last week it was reported that U.S. military advisers in Honduras have risen above 100.

Paz Barnica said the actual number of advisers is no more than 20 and any additional ones had come only briefly for "mechanical" tasks "of no significance."

The plan calls for "international supervision and inspection, to which Honduras is determined to submit." In an interview, Paz Barnica said "that role could be assumed by the OAS," which has played virtually no role in the Central American crisis so far.

Paz Barnica, who worked for the OAS Human Rights Commission before assuming the foreign minister post under President Roberto Suazo Cordova, said that the Honduran armed forces helped prepare the proposal and supported it. He also said he expected support from Guatemala, although the coup there this week precluded any contacts yet.