Career Foreign Service officers at Israel's embassy in Washington are seeking to block the appointment of an aide to Prime Minister Menachem Begin to a key embassy post.

The dispute over the job in Washington is part of a larger struggle between the Begin government and the Israeli Foreign Service, whose members charge that they are being undermined by the appointment of nonprofessionals to important diplomatic posts and who are seeking improvements in their wages and working conditions.

In a cable addressed to Foreign Ministry Director General David Kimche last week, the 10 professional Foreign Service officers who work at the embassy in Washington strongly objected to the expected nomination of Yoram Ettinger, who works in the information section of Begin's office, to the post of minister for information at the embassy.

"Nominations of outsiders and their incorporation into the ranks of the Foreign Service hurt our chances for promotion and damage the morale of trusted and competent employes," the Washington embassy staff complained in the cable.

The post of minister for information is one of the highest ranking at the embassy and involves the coordination of Israeli government information policy in Washington and at the 11 Israeli consulates around the United States. The job is currently held by Harry Horowitz, who was also an adviser to Begin.

According to Foreign Ministry documents, when Horowitz was appointed three years ago Foreign Minister Yitzhak Shamir pledged in writing that the naming of a member of Begin's staff to the Washington job would not set a "precedent." Now, Foreign Ministry employes charge, Shamir is about to break that promise.

"It's becoming a trend," said one official. "There are people in the ministry who can do the job."

Ettinger, who was described as an "unqualified outsider" by the leaders of the Foreign Service employes' union in a letter to Shamir, has worked in government for a number of years but has virtually no foreign policy experience.

The fact that Foreign Ministry officials made their complaints in the Ettinger case public reflected their apparently deepening disillusionment with Shamir's leadership. Officials long have complained privately of alleged political interference in Foreign Service appointments and for months have been waging a fruitless campaign for higher wages.