The family at the centre of a row over diplomatic immunity after their son died in a car crash described a meeting with Foreign Secretary Dominic Raab as feeling like a "publicity stunt".
Harry Dunn, 19, died in a crash with a Volvo in Northamptonshire on 27 August.
American diplomat's wife Anne Sacoolas, suspected of driving the other vehicle, later left the UK to return to the US.
Boris Johnson has spoken to President Trump who told a press briefing Harry's death was a "terrible accident".
Police have said CCTV of the crash which killed the teenager shows the Volvo travelling on the wrong side of the road.
Speaking after his conversation with the prime minister, President Trump said: "The woman was driving on the wrong side of the road, and that can happen.
"You know, those are the opposite roads, that happens. I won't say it ever happened to me, but it did.
"So a young man was killed, the person that was driving the automobile has diplomatic immunity, we're going to speak to her very shortly and see if we can do something where they meet.
"It was an accident, it was a terrible accident."
After meeting the foreign secretary, Harry's mother Charlotte Charles said she felt "let down" by both the UK and US governments.
She said: "I can't really see the point as to why we were invited to see Dominic Raab. We are no further forward than where we were this time last week.
"Part of me is feeling like it was just a publicity stunt on the UK Government side to show they are trying to help.
"But, although he is engaging with us, we have no answers. We are really frustrated that we could spend half an hour or more with him and just come out with nothing."
Together with Harry's father Tim Dunn, she met Mr Raab in the hope he would urge the US to waive Ms Sacoolas' diplomatic immunity.
Mr Dunn said: "I felt extremely let down by the Government today, or by the Foreign and Commonwealth Office.
"I'm deeply, deeply disappointed that they think it's okay to kill a young lad on his bike and they can just walk away.
"I don't think the government or the Commonwealth Office have any clout to do anything."
Number 10 said the Prime Minister urged US President Donald Trump to reconsider the decision to allow Ms Sacoolas immunity in order that "the individual involved can return to the UK, cooperate with police and allow Harry's family to receive justice".
Downing Street said the "leaders agreed to work together to find a way forward as soon as possible" during their conversation on Wednesday evening.
Following the meeting with Harry's parents, the foreign secretary said: "I share the frustration of Harry's mother and father.
"They have lost their son and the justice process is not being allowed to properly run its course."
Meanwhile, Ms Charles urged Ms Sacoolas to do the "humane thing to do and get on a plane and come back".
Their lawyer Radd Seiger said they were in talks to launch a civil case against Ms Sacoolas and they were "going to Washington soon to help us get that justice for Harry".
He also invited the US President to meet the family about the case.
"If meeting with President Trump would help us get a step closer to seek justice for Harry, to get justice for that boy who died that night needlessly, one of the most wonderful kids in our community, if that's what it takes then I will extend an invitation now to President Trump.
"Meet us. Let's have a chat. Nobody wants to litigate."
Mr Johnson had already urged the US to reconsider its decision to allow Ms Sacoolas immunity, while Mr Raab has previously spoken to the US ambassador and Secretary of State Mike Pompeo.
Northamptonshire's chief constable and its police and crime commissioner have also urged the Americans to waive Ms Sacoolas's diplomatic immunity.
What is diplomatic immunity?
About 23,000 individuals in the UK have diplomatic immunity, a status reserved for foreign diplomats and their families, as long as they don't have British citizenship.
It is granted by the 1961 Vienna Convention and means that, in theory, diplomats cannot face court proceedings for any crime or civil case.
The convention also states that those entitled to immunity are expected to obey the law.
Where crimes are committed, the Foreign Office can ask a foreign government to waive immunity where they feel it is appropriate.
Diplomatic immunity is by no means restricted to those named on the Diplomatic List from the Foreign and Commonwealth Office.
Drivers, cooks and other support staff whose names do not appear, but have been accredited to Britain ("the receiving state") have the same diplomatic status and immunity as those who are listed.
Equally, there are a number of foreign nationals in Britain attached to international organizations who have the same status and protection.
Harry Dunn died after his Kawasaki motorcycle was in a crash with a black Volvo XC90 in Croughton, close to an RAF base.
He was taken to Radcliffe Hospital in Oxford where he died.
Chief Constable Nick Adderley said "based on CCTV evidence", officers knew that "a vehicle alighted from the RAF base at Croughton" and was "on the wrong side of the road".
He said the suspect, Ms Sacoolas, had "engaged fully" following the crash and said "she had no plans to leave the country in the near future".
However, she then left for the United States and has not returned.