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 disappearance of Jamal Khashoggi, a former royal insider, has strained U.S.-Saudi ties and sparked international outrage. He was last seen entering the Saudi consulate in Istanbul two weeks ago. Saudi Arabia has denied any role in his disappearance.

Mr. Trump has put Saudi Arabia and its crown prince at the center of a Middle East policy aimed at challenging Iran, Riyadh’s main rival in the region, brokering an end to the Palestinian-Israeli crisis and countering extremism. Mr. Trump chose Saudi Arabia as the site of his first overseas presidential trip. The U.S. leader has emphasized the importance for U.S. jobs of the Saudi’s purchasing of billions of dollars of U.S. military hardware.

In an interview with the Associated Press Tuesday, Mr. Trump compared the allegations that Saudi agents had killed Mr. Khashoggi to the accusations of sexual misconduct against Supreme Court Justice Brett Kavanaugh, who was sworn in last week after a bruising confirmation process. “Here we go again with, you know, you’re guilty until proven innocent,” Mr. Trump said. “We just went through that with Justice Kavanaugh and he was innocent all the way as far as I’m concerned.”