DAKAR, Senegal — The eccentric president of Gambia has warned that he will carry out a mass execution of prisoners by mid-September, a threat local journalists and opposition officials said must be taken seriously given his unpredictability and long record of human rights abuses.

President Yahya Jammeh, a former wrestler and army colonel who seized power in the tiny nation 18 years ago, said in a widely reported speech this week that “by the middle of next month, all the death sentences would have been carried out to the letter.” Amnesty International says 47 people are on death row, including a number of former government officials convicted of “treason” for supposedly plotting to overthrow Mr. Jammeh’s government. The death sentence in Gambia is usually carried out by hanging.

Amnesty said nine prisoners had already been taken from their cells and executed, according to “credible reports” it said it had received. The organization said that the nine were executed Thursday night, and that three of them had previously been convicted of treason.

The editor of the leading opposition newspaper, Foroyaa — himself a regular target of government persecution — said he could not yet confirm the executions late on Friday. “What is clear is that nine people were removed from where they were” on death row, said the editor, Sam Sarr. “We do not yet know what happened to them.”