An American Christian couple accused of starving their adopted daughter to death were acquitted on Sunday, but were stopped by security when they tried to leave Qatar.

Matthew and Grace Huang's eight-year-old daughter Gloria died in January 2013. The parents were arrested the day after her death and subsequently charged with murder.

The case has been riddled with confusion. The Huangs were eventually convicted in March 2014 of exposing Gloria to danger, but not deliberately starving her. They were sentenced to three years in prison.

The ruling was overturned on Sunday at an appeal court, where it was found that the lower courts had made numerous mistakes.

But when the couple tried to leave the country on Sunday evening, they were told at Doha airport that a new arrest warrant had been issued and their passports were taken from them, the family's representative, Eric Volz, told Reuters.

"This is sort of a spiteful move," Volz said.

US Ambassador Dana Shell Smith was at the airport to meet the Huangs and tried to intervene with Qatari authorities, Associated Press reports.

The Huangs, originally from Los Angeles, moved to Qatar in 2012 for Matthew's work as an engineer.

The parents' supporters say that the accusation stemmed from cultural differences and discrimination against their inter-racial family. Gloria and her two brothers were adopted from Africa.

The medical report said that Gloria died from an irreversible loss of body mass and dehydration. The prosecution alleged that her parents had starved her and kept her in a locked room.

The family maintain that she had had struggled with malnutrition since they adopted her from Ghana aged four.

On a website campaigning for the Huangs' release, Gloria's erratic eating patterns are described: "From time to time she [Gloria] would exhibit an eating disorder – common among children with backgrounds similar to hers – where she would refuse food for days at a time and then eat more than an adult. Other times she would eat food from the garbage even when she had healthy food available. Yet most of the time she was vibrant and seemingly healthy. She then died suddenly in the midst of one of her cycles of refusing to eat."

US Secretary of State John Kerry said in a statement: "The thoroughly documented findings of the court clearly establish the Huangs' innocence.

"The 22 long months of court proceedings following their daughter's tragic death have compounded the tragedy for the Huang family, and it is time now, as the Appeals Court stated, to let the Huangs return home."

The Huangs' two sons were originally placed in an orphanage, but have since returned to the US.