TEL AVIV—Israeli Prime Minister Benjamin Netanyahu on Thursday condemned comments previously made by his newly appointed chief of public diplomacy that belittled Israeli leaders, insulted Secretary of State John Kerry and suggested President Barack Obama was anti-Semitic.

"I have just read Dr. Ran Baratz’s posts on the Internet,” Mr. Netanyahu said in a statement, less than 24 hours after the appointment. “Those posts are totally unacceptable and in no way reflect my positions or the policies of the Government of Israel.”

Mr. Baratz, whose appointment is still subject to a signoff from Israel’s cabinet, apologized Thursday and said he had failed to inform Mr. Netanyahu about the social-media posts before taking the job.

“These postings were written hastily and sometimes humorously, in a manner appropriate for a private person writing on the Internet,” he said in a statement. “It is clear to me that in a government position one must act and express oneself differently.”

Mr. Netanyahu’s statement comes as he and Mr. Obama prepare to meet for the first time in more than a year on Monday in Washington.