The Italian couple, Maria Chiara Bornacin and Alessandro Giusto, say they are keeping the girl, who has been given the pseudonym "Maria," in hiding because she has suffered repeated sexual and physical abuse at the orphanage in Belarus where she spends most of the year.
Bornacin has said she would rather go to prison than see Maria return to Belarus.
The Belarusian authorities have downplayed the abuse claims. Deputy Education Minister Tatsyana Kavalyova denies the girl has been abused in the orphanage.
Belarus's ambassador to Italy, Alyaksey Skrypko, told RFE/RL's Belarus Service the matter will be looked into. "We will definitely verify these allegations by the Italian family and will definitely conduct a completely thorough investigation of the circumstances that became a basis for such allegations," Skrypko said.
Meanwhile, employees at the orphanage in Vileyka, 100 kilometers northwest of Minsk, have been reluctant to talk. One staff member, who only gave his name as Mikalay, said it was too early to comment on the allegations.
"There is some doubt, indeed. You see, it is a [tricky] situation. I think it will soon be unraveled, and then it will be possible to make some comments. Now one should be cautious, because there are problems and they should be resolved quietly," Mikalay said.
Maria first stayed with the Italian couple in 2003. Since then, she has spent several summers in Italy as part of a program launched after the 1986 Chornobyl nuclear disaster to send children to healthier climates abroad.
The couple says they first noticed bruising and signs of abuse on Maria's body in 2004 and alerted doctors.
But they say they were forced to take dramatic measures this summer, after Maria tried to drown herself in the sea, apparently out of dread at returning to the orphanage.
Rather than returning her to Belarus on schedule earlier this month, the couple put Maria in a secret hiding place and have refused to reveal her location.
The case has caused a diplomatic row between Italy and Belarus.
The head of the Belarusian Foreign Ministry's press office, Maryya Vanshyna, said in a recent statement that the Italian couple's decision to hide the girl "imperils the interests of hundreds of Belarusian children from Chornobyl-affected areas."
Vanshyna also said the Italian authorities have accepted "official guarantees" from Belarus that Maria will be given psychological and medical help when she returns home.
Despite those assurances, many Italians support the couple, saying they are just acting in the child's interest.
Father Klaudi, a priest from Radio Vatican, says he doesn't agree with some allegations that the couple have manipulated the girl.
"I don't believe it to be true that the girl was prompted [by the Italian couple] as to what she should say. The whole village supports the parents," Klaudi says.
Cogoleto's parish priest, Father Danilo Grillo, is one of those. He told Britain's "The Guardian" daily that he was ready to sidestep the law and give the girl refuge in the church.
But despite reported negotiations between the Italian and Belarusian authorities, the case appears to be no closer to resolution.
Maria remains in hiding and her foster parents, who are under investigation for kidnapping of a minor, have not been arrested.
(RFE/RL's Belarus Service contributed to this report.)