Comments come as the US secretary of state embarks on a tour of Arab capitals

The US does not believe the official Saudi version of the murder of the Washington Post columnist and dissident Jamal Khashoggi is credible, a senior administration official has said.

The official was speaking to the press before the secretary of state, Mike Pompeo, embarks next week on an tour of eight Arab capitals, seeking to shore up support for US policy, and to reassure allies that the US is not abandoning the region despite Donald Trump’s order for the withdrawal of American troops from Syria.

In Riyadh, Pompeo will talk to the Saudi leadership about the Khashoggi murder and the war in Yemen, urging support for the peace process begun in Stockholm last month.

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The Saudi authorities have charged 12 men for the killing on 2 October in the kingdom’s Istanbul consulate. Five of the defendants face the death penalty if convicted.

“The Saudis should have a credible narrative of what happens in the consulate and afterwards,” the senior official said. “I don’t think from our point of view, the narrative emerging from the Saudis or the legal process there has yet hit that threshold of credibility and accountability.”

He added: “We have continued to work this issue with the Saudis, underscoring that it’s in their interest to pursue this as aggressively as they can to get this albatross off their backs and to get out from under the shadow of this incident which has caused such an outcry.”

The CIA has assessed that the killing was ordered by the crown prince, Mohammed bin Salman, a conclusion endorsed by the US Senate, but Trump and his senior officials, including Pompeo, have continued to insist the evidence against the prince, a key partner in the administration’s Middle East policy, is inconclusive.

Facebook Twitter Pinterest Mike Pompeo is seeking to reassure allies that the US is not abandoning the region. Photograph: Mandel Ngan/AFP/Getty Images

A spokeswoman for the UN’s top human rights official, Michelle Bachelet, said that the trial in Riyadh does not meet the requirements of an independent and international enquiry.

“We, as you know, have been pressing for justice in the Khashoggi case for months now. We have been calling for an investigation, an independent investigation, with international involvement, and this has not happened yet,” the spokeswoman, Ravina Shamdasani, said on Friday.

Pompeo’s tour of the region will also include stops in Jordan, Egypt (where he will make a speech on US policy), Bahrain, Abu Dhabi, Qatar, Oman and Kuwait.

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Another senior official briefing the press before the trip, said that the secretary of state aimed to debunk a “false narrative” that the US was leaving the region, in the wake of Trump’s order for the withdrawal of an estimated 2,000 US troops in Syria.

He said there was no timeline for the withdraw and that it would be “heavily coordinated” with US allies in the region so as not to let up pressure on Isis, or leave a vacuum in areas of northern and southern Syria where US troops have been operating.

Asked about reports of plans to raise an Arab force to take over from US troops, the official said that the US is “exploring a variety of options ... but we don’t have any plans right now to facilitate Arab forces going into Syria”