MANAGUA, Nicaragua (AP) _ Contra rebels today rejected a Sandinista government proposal to reopen peace talks and said the offer was linked to Washington's consideration of military aid for the rebels.

''Every time the Americans talk of military aid, they (the Sandinistas) talk of negotiation,'' Contra leader Roberto Ferrey said in a telephone interview from Costa Rica.

The Reagan administration, which has supported the rebels since 1981, is considering whether to ask Congress for renewed military aid.

President Daniel Ortega called on the rebels Sunday to hold a fifth round of talks before a temporary cease-fire expires at the end of June. He proposed that the meeting begin June 26.

The two sides have made little headway since signing an agreement March 23 to seek a negotiated settlement to the almost seven-year war.

The Contra rejection of talks next week did not rule out further negotiations. All of the meetings have been marked by haggling over dates and places.

During the last meeting, the Sandinistas presented their most liberal proposal.

Under the plan, the rebels would send up to eight delegates to take part in internal talks with other political opposition party members to define such issues as freedom of expression, electoral laws and the separation of party and state.

The Contras said the government plan did not assure that the democratic measures would be enacted.

They also called for a constituent assembly, a new supreme court and permission for soldiers to leave military service should they so desire.

The rebels also demanded immediate political amnesty for more than 3,000 prisoners.