An appeals court in Qatar on Sunday overturned the convictions of a Los Angeles couple who were sentenced to three years in jail for the death of their adopted African-born daughter, but the government was refusing to let the couple leave the country, a spokesman for the family said.

Matthew and Grace Huang were trying to return to the US after the tribunal found the lower court had made numerous errors, family spokesman Eric Volz told Reuters. But as soon as they arrived at the airport in Doha, their passports were seized and they were told a new arrest warrant had been issued for them, Volz said.

The US State Department, which could not be reached for comment on Sunday, had a representative at the airport and was working to help the couple, Volz said.

"It's not over. It's escalating very quickly," Volz said. "This is a showdown."

A representative of the Qatari government could not be reached immediately.

The Huangs were arrested in January 2013 after their eight-year-old daughter, Gloria, died unexpectedly.

An autopsy found that the girl had died of "cachexia and dehydration," and a prosecutor charged the couple with "murder with intent by forced starvation." Cachexia is an irreversible loss of body mass. They were convicted in April.

The couple argued that Gloria had been suffering from malnutrition-related diseases since they adopted her from Ghana at the age of four, and that the Qatari authorities had failed to acknowledge this.

The US State Department has been working on the couple's behalf to appeal against the convictions. It had said that "cultural misunderstandings may have led to an unfair trial."

A website created to publicise the case said Matthew Huang, a Stanford University-trained engineer, had moved to Qatar with his wife and their three young children in 2012 to help oversee a big infrastructure project related to the 2022 soccer World Cup.

Gloria died on January 15, 2013, and Qatari police charged the couple the next day. The family's two other adopted children were placed in an orphanage initially, but have since returned to the US.

The Huangs spent nearly a year in prison before being released on their own recognizance, awaiting appeal.  - Reuters