RIO DE JANEIRO — Edward J. Snowden, the former contractor for the National Security Agency now living temporarily in Russia, said in comments published on Tuesday that he was prepared to assist Brazilian investigations into American spying in Brazil. But he said he could not speak freely until a country granted him permanent political asylum, which he requested from Brazil months ago.

Mr. Snowden, whose disclosures of N.S.A. surveillance practices have shaken Washington’s relations with an array of countries, made his comments in an “open letter” published in the Brazilian newspaper, Folha de São Paulo, in which he described the agency’s activities as potentially “the greatest human rights challenge of our time.”

Brazil, a leading target of the N.S.A.’s activities, has already reacted angrily over the spying, which included surveillance of President Dilma Rousseff, her inner circle of senior advisers and Petrobras, Brazil’s national oil company. Ms. Rousseff called off a state visit to Washington in October over the revelations of the N.S.A.’s operations in Brazil.

Since then, Brazilian legislators have pressed ahead with inquiries into spying by the United States, relying to a large degree on news reports and testimony by Glenn Greenwald, the American journalist to whom Mr. Snowden leaked N.S.A. documents. David Miranda, the domestic partner of Mr. Greenwald, who lives in Rio de Janeiro, has helped lead an effort to obtain asylum in Brazil for Mr. Snowden, who is now in Russia on a one-year visa.