The US ambassador to the United Nations, who has announced she is leaving her post, has blamed Saudi Arabia's powerful crown prince for the murder of journalist Jamal Khashoggi, breaking with the position of her boss Donald Trump.

Nikki Haley made the accusation against Mohammed bin Salman (MBS) on Wednesday in a tell-all interview with NBC.

"It was the Saudi government, and MBS is the head of the Saudi government," Haley said when asked about Khashoggi's murder.

"So they are all responsible, and they don't get a pass, not an individual, not the government - they don't get a pass," she added.

Haley announced in October that she would leave the administration at the end of 2018.

The comments come as US President Donald Trump said he would continue to stand by the crown prince despite increasing pressure from US authorities, including the CIA and a number of senators.

Trump downplayed concerns surrounding Prince Mohammed's involvement in the brutal killing of Khashoggi in Riyadh's consulate in Istanbul on October 2.

A member of the US House of Representatives said on Wednesday that hearings would be held after the first of the year on all aspects of Saudi behavior, including the killing of Khashoggi.

Saudi Arabia has denied that the crown prince was involved and has arrested 21 people in connection with the killing.

Saudi journalist Jamal Khashoggi, who was murdered in October at his country's Istanbul consulate, was named Time magazine's "Person of the Year" on Tuesday, an honour he shared with other persecuted journalists dubbed as "guardians" of the truth.