Donald Trump has dismissed a United Nations request for the FBI to investigate the murder of the dissident journalist Jamal Khashoggi, suggesting it would jeopardise American weapons sales to Saudi Arabia.

A report on Khashoggi’s assassination published last week by the UN special rapporteur on extrajudicial killings said the US should open an FBI inquiry and “pursue criminal prosecutions within the United States, as appropriate”.

But in an interview broadcast by NBC’s Meet the Press on Sunday Trump brushed the proposal aside.

Asked if he would allow the FBI to investigate, Trump said: “I think it’s been heavily investigated.”

Asked who had investigated, the president replied: “By everybody. I mean, I’ve seen so many different reports.”

Khashoggi, 59, was a US resident who wrote for the Washington Post. He was killed and dismembered after he entered the Saudi consulate in Istanbul in October last year, seeking paperwork he needed in order to marry.

The UN report contains disturbing details of conversations between unidentified Saudis before and after Khashoggi’s arrival, based on transcripts provided by Turkish authorities.

Khashoggi is referred to as a “sacrificial animal”. One Saudi official is quoted as asking whether it would “be possible to put the trunk in a bag”. Another replies: “No. Too heavy. It is not a problem. The body is heavy. First time I cut on the ground. If we take plastic bags and cut it into pieces, it will be finished. We will wrap each of them.”

Transcripts of conversations after Khashoggi’s arrival at the consulate include the journalist saying: “There is a towel here. Are you going to give me drugs?”

The reply: “We will anaesthetise you.”

The UN special rapporteur blamed the Saudi government for the murder and said there was credible evidence that Crown Prince Mohammed bin Salman and other senior officials were responsible.

Trump told NBC the murder “really didn’t come up” in a call this week with the prince, a key ally of the president’s senior adviser and son-in-law Jared Kushner, who among other responsibilities is charged with implementing a plan for peace between Israel and the Palestinians.

Trump also suggested Saudi Arabia was no worse than other states in the Middle East, which he called a “vicious, hostile place”, adding: “Look at Iran, look at other countries, I won’t mention names.”

The president then cited a drastically overinflated figure for Saudi spending on US weapons that fact-checkers have previously noted does not match the official record.

“I only say they spend $400bn to $450bn over a period of time, all money, all jobs, buying equipment,” Trump said.

In fact Saudi Arabia last year signed “letters of offer and acceptance” for $14.5bn in military purchases from the US.

The Senate last week voted to block the Trump administration selling arms to Saudi Arabia, seven Republicans joining Democrats to pass the measure. Trump has pledged to use his presidential veto and push on with the sales.

While denying he was saying such purchases were “the price” for Khashoggi’s murder, Trump on Sunday defended his consideration of arms sales in responding to the assassination.

“I’m not like a fool that says, ‘We don’t want to do business with them,’” Trump said. “And by the way, if they don’t do business with us, you know what they do? They’ll do business with the Russians or with the Chinese …

“We make the best equipment in the world, but they will buy great equipment from Russia and from China.”

Prince Mohammed has denied responsibility for the murder. Eleven suspects are on trial in Riyadh, five of them facing the death penalty.

Confronted with gruesome details from the UN report this week, Saudi minister of state for foreign affairs Adel al-Jubeir told CNN the report was “flawed”.

“We know this was a rogue operation that was not authorised,” he said, “we know that a crime was committed, we have people in jail and on trial as we speak. It’s a gruesome murder that happened outside authorities and for which the people who committed it will be punished … this should never have happened.”

Al-Jubeir also said “mechanisms” were being “put in place to ensure this does not happen again”.