The Reagan administration yesterday called Nicaragua's endorsement of a proposed Central America peace treaty "a propaganda ploy" and said the treaty provisions, as they now stand, cannot bring peace and security to the region.

At a briefing to explain the U.S. position, a senior State Department official said the United States is urging its Central American allies -- El Salvador, Honduras and Costa Rica -- not to reject the proposed pact but to strengthen its provisions in ways that will genuinely limit the size of regional military forces and restrict military activities by countries outside the area.

"We are looking for a document that will bring peace and security to the region and not just a propaganda ploy," said the official, who cannot be identified under the rules of the briefing.

At issue is the comprehensive treaty proposed last month by the Contadora group -- Mexico, Colombia, Venezuela and Panama -- that has been trying to mediate a peaceful resolution of the tensions between the leftist Sandinista government in Nicaragua and the other Central American countries.

Two weeks ago, Nicaragua unexpectedly agreed to sign the latest draft of the treaty, and the other countries followed with unofficial expressions of support. That allowed Nicaragua to cast itself as a peacemaker and diverted attention from a growing debate about whether Nicaragua's presidential elections, scheduled for Nov. 4, will be democratic.

It also created difficulties for the administration, which has publicly supported the idea of solving the Central America crisis within the Contadora framework and which had insisted that Nicaragua was trying to block progress toward a treaty.

However, it has become clear in recent days that the United States is pressing its three Central American allies to demand changes in the treaty text. That has exposed the United States to charges that it never really meant to support Contadora and is not really interested in accommodation with Nicaragua.

That was disputed by the senior official who said that the United States regards much of what has been accomplished by Contadora so far as "a significant achievement." But, despite indications from the region that the participants had been ready to sign before Washington put on pressure, the official insisted that, except for Nicaragua, the other Central American countries always have regarded the current treaty text as only a draft that requires more work to plug "obvious loopholes."

The official said the United States objects to the present treaty language because it does not provide adequate mechanisms for verifying that countries of Central America are not interfering in the affairs of their neighbors. He also said the treaty, as presently worded, would permit Nicaragua, which has the largest army in the region, to avoid reducing its size while insisting that the United States immediately halt its military aid to El Salvador and Honduras.

Lastly, the official contended, the treaty's present language about "democratization" of the region is so imprecise that Nicaragua could use it to legitimatize what the official called its "patently fraudulent" upcoming elections rather than heed demands for a new and fairer election system.

The official said the United States believes these concerns are shared by the other Central American countries, and he predicted that unless they are resolved, it will be virtually impossible to achieve a treaty.