London-based Amnesty International has written to Secretary of State George P. Shultz asking for assurances that any American assistance to the contras fighting the Nicaraguan government will not lead to human rights abuses. Amnesty International said it believes that Washington is not concerned with such abuses. The letter details alleged torture and execution of civilians and military personnel by the rebels. The letter, sent last October, was disclosed this week. The human rights group said it had not received a reply.