The association of correspondents who cover the State Department sent a letter yesterday to President Reagan's national security adviser, William P. Clark, expressing "serious concern" about new White House directives restricting contacts between reporters and government officials.

The letter, signed by association president Barrie Dunsmore of ABC-TV, said the change "is already having a chilling impact," with many officials whom reporters have known for years "now reluctant to discuss or explain even basic points of policy."

A sizable number of complaints are reaching Clark's office over the restrictions ordered by Reagan on Jan. 12, administration officials said privately.

Clark's office, one official said, has been made aware of many complaints by reporters having trouble contacting middle-level government officials even on the most routine matters. There have been similar complaints about appointments being canceled, telephone calls not returned, and secretaries who will not put calls through but insist that all inquiries go to a press office, a tactic that frequently results in information that is late or not helpful.

"If the only news we can report is based on sanitized handouts, served up by government press offices, neither public understanding nor public support are likely to be enhanced," the Dunsmore letter said.

Jack Landau, head of the Reporters' Committee for Freedom of the Press, said he could see "no other reason for the White House to look over the shoulder of every policy maker who talks with the press except to make sure that whatever information gets out makes the administration look good." The new restrictions, Landau said, will "make officials less honest and less open."