Nicaragua's deputy foreign minister today delivered a blistering attack on the United States, charging that it was "obsessed with the desire to destroy us" and saying that history would judge President Reagan harshly for deaths in Nicaragua caused by U.S.-backed "freedom fighters."

In an address here to the Organization of American States, Vice Foreign Minister Nora Astorga also charged that the United States had exerted pressure on its Central American allies to demand unacceptable alterations to the Contadora draft treaty.

At the same time, the four Contadora countries issued a communique on their talks in Brasilia saying proposals made by three Central American nations at Tegucigalpa to alter the Contadora Act of Sept. 9 represented a valuable contribution. These demands, however, could alter the balance achieved previously and only some would be adopted, the communique said.

The Reagan administration has criticized and its Central American allies have sought changes in a peace proposal by Contadora nations Mexico, Colombia, Venezuela and Panama that included a freeze on armaments and a ban on foreign military bases and advisers in the region.

The communique made no reference to calls by U.S. Secretary of State George P. Shultz on Monday for verifiable control mechanisms and a timetable for peace in the region.

"Since the beginning the Reagan administration has been determined to torpedo Contadora, all the while declaring its support," said Deputy Foreign Minister Astorga.

The Nicaraguan said proof of the U.S. intention was shown by a recently leaked National Security Council document which says the U.S. "effectively blocked" a draft Contadora proposal.