The Reagan administration has sent a series of messages to Philippine President Ferdinand Marcos calling for any new balloting there to be clean and fair but is taking no position on the possibility of accelerated presidential elections, according to State Department sources.

U.S. Ambassador Stephen Bosworth is reported to have expressed U.S. concern about the fairness of the election process, especially since Marcos last month named three persons believed to be Marcos loyalists to the national Commission on Elections. As a result, six of the seven commissioners are widely considered to be Marcos loyalists.

Bosworth is also said to have conveyed U.S. concern about the uncertain status of the National Citizens Movement for Free Elections (NAMFREL), a watchdog group whose poll-watching was widely credited with bringing an unusual measure of honesty to the May 1984 legislative elections.

NAMFREL mobilized 300,000 citizen volunteers in its casting and counting of ballots, but Marcos has withdrawn its recognition as an official watchdog group, and its future is unclear.

Such actions as these, in the view of State Department sources, raise serious questions about the fairness and credibility of forthcoming national elections. The ballot box represents the main hope of the U.S. administration for an evolutionary rather than revolutionary transition from Marcos' increasingly embattled rule.

The next scheduled national elections in the Philippines are municipal and provincial elections set for May. These are considered of critical importance in setting the stage for the presidential elections in 1987, when Marcos' six-year term ends.

In recent weeks, however, Marcos has said he is considering early presidential elections -- sometimes called "snap" elections. Manila has been rife with rumors that Marcos will arrange to hold the balloting this year in order to catch his opposition unprepared and maximize his reelection chances.

State Department sources said the Reagan administration has taken no position and has transmitted no messages on the issue of "snap" elections. "It's not an issue the United States should weigh in on," said one official. He added that it is difficult to foresee how early presidential elections would affect the political process and that the position of the opposition is far from clear.

The sources continued to deny reports that the United States has urged "snap" presidential elections, including a Newsweek report that Central Intelligence Agency Director William J. Casey pushed for the early balloting during a conversation with Marcos in Manila in May.

Marcos' declining political and physical health is the central problem in the deteriorating Philippine situation, in the view of worried U.S. officials.

A U.S. source familiar with detailed reports from Manila said Marcos, 68, underwent a kidney transplant last fall in an effort to stop his physical decline. One official referred to Marcos as "a walking zombie" and said the Philippine leader simply does not have the physical ability or the political prestige to confront his problems as effectively as he once could.