A Saudi Arabian man accused of murder in the US is unlikely to face justice because he has fled the country, a US State Department official has said.

Abdulrahman Sameer Noorah is believed to have fled the US state of Oregon with the help of Saudi officials and returned to his home country.

In a condolence letter to Oregon Senator Ron Wyden, a senior US diplomat wrote that extradition was unlikely.

The letter does not mention what effort the US will go to to seek extradition.

"The United States and Saudi Arabia do not have a bilateral extradition treaty, and Saudi Arabia does not extradite its nationals to the United States," wrote Mary Elizabeth Taylor, the State Department's assistant secretary of legislative affairs, in a letter provided by Senator Wyden's office to BBC News.

"Therefore, the law enforcement options available are limited."

The letter included condolences to the family of 15-year-old Fallon Smart - who Mr Noorah is accused of killing in a hit-and-run crash in 2016 - and said the State Department "fully understands their desire to see Mr Noorah prosecuted".

Ms Taylor said that through "police-to-police" channels, officials had learned that Mr Noorah had returned to Saudi Arabia. But she wrote that US officials had "no concrete, credible evidence as to how Mr Noorah effected his escape".

The Oregonian newspaper reported at least 16 cases around the US and Canada in which Saudi university students vanished while facing criminal charges, even after surrendering their passports to police, as Mr Noorah did.

After Mr Noorah's arrest, the Saudi consulate provided $100,000 (75,000) to bail him out of jail and provided him with private defence lawyers.

Mr Noorah, who was then 21, had been receiving a stipend from the Saudi government to study in the US before the alleged crime. He was last seen leaving his Portland neighbourhood in a black SUV.

US federal authorities later told the Oregonian that he used an illicit passport to fly out of the US on a private plane.

How have US officials reacted?

In a statement, Senator Wyden accused Secretary of State Mike Pompeo of "covering for an authoritarian regime that is exporting lawlessness around the globe".

"Secretary Pompeo is proud to brag about America's military and economic dominance, but he apparently believes the State Department is powerless to stand up to Saudi Arabia's long pattern of apparently helping criminal suspects escape US justice.

"That is not good enough for the victims of these violent crimes in Oregon and across the country."

Chris Larsen, an lawyer for Fallon Smart's family, told the Oregonian that the family was "disappointed with the State Department's apparent unwillingness to use its considerable diplomatic power to work with the Saudi government to return Mr Noorah to Oregon to face trial for his crimes".

"While we appreciate Secretary Pompeo's condolences, what we need is a commitment by this administration to act: to negotiate with the King of Saudi Arabia for the return of Mr Noorah.

"We understand that while 'the law enforcement options are limited', the Department of State options are not so limited," he said.