ISTANBUL—Saudi operatives beat, drugged, killed and dismembered a dissident Saudi journalist in the presence of the kingdom’s top diplomat in Istanbul, Turkish officials said Tuesday, as Washington urged Riyadh to provide answers.

President Trump cautioned that Saudi Arabia should be considered innocent until proven guilty. His secretary of state, Mike Pompeo, on a visit to the kingdom, said Saudi leaders had “strongly denied” involvement and were conducting “a serious and credible investigation.”

The...