Former ambassador Robert White's charge that increased U.S. military aid to El Salvador is playing into the hands of antidemocratic rightist forces was disavowed by the State Department yesterday in a way that indicated the Reagan administration plans to force White out of the Foreign Service.

Department spokesman William Dyess, disputing White's testimony Wednesday before a House subcommittee, pointedly noted that he could not recall another instance in his 20-year diplomatic career when a senior Foreign Service officer had disagreed publicly with the policy of the administration in office.

White was summarily dismissed by Secretary of State Alexander M. Haig Jr. last month after 11 months as ambassador. Washington sources said yesterday that Deane R. Hinton, a veteran Foreign Service officer who was chief of the State Department's bureau of economic and business affairs in the Carter administration, has been picked to be White's successor. Hinton previously served in Guatemala and Chile.

In his testimony Wednesday, White contended that the weekend leftist guerrillas in El Salvador's civil are not a serious threat and that providing new military equipment will allow rightist-influenced security forces to "assassinate and kill in a totally uncontrolled way."

Questions about White's charges led to an exchange between Dyess and reporters at yesterday's State Department news briefing that went on for almost 1 1/2 hours and was characterized by an unusual tone of barely controlled emotion on the part of some questioners.

The nature of the questions showed that President Reagan's decision to make El Salvador a testing ground for his determination to halt communist support of insurgent movements remains a highly sensitive issue characterized by fears that the administration is prepared to back rightist forces in the Third World. Some of the questions also centered on whether the isolation and disfavor in which White now finds himself indicate that the administration will permit no dissent from its policies.

Dyess, presenting the administration view, said, "There were differences between this administration and Ambassador White. . . . Where we disagree is where the immediate, principal threat is coming from. He seems to think it is coming from the right. We think it is from the leftist insurgents.

"I understand Ambassador White feels very strongly about the issue. I don't know him personally, but I understand he's somewhat emotional about it, and when a person has very strongly held views, maybe the only thing to do is express them."

Later, Dyess said his comments about White's emotionalism were not intended to be "pejorative." However, a senior White House official, speaking on background characterized White as a "sorehead" and implied that there is no place for him in the conduct of the administration's foreign policy.

On the record, Dyess would say only that several new assignments included jobs as an inspector and others below the level of someone who had held ambassadorial rank. Under Foreign Service regulations, if White refuses what is offered, he will be separated from the service.