American and South Sudanese diplomats have been summoned to Khartoum after Christian Meriam Ibrahim and her family were detained.
On June 24, an appeals court overturned Ms Ibrahim’s death sentence and she was freed from prison.
But Ms Ibrahim and her family were apprehended at Khartoum Airport and detained, reportedly over issues with travel documents.
Her husband had told media they were trying to get to the United States.
Ms Ibrahim, who had a Muslim father and an Ethiopian Orthodox mother, had been convicted of apostasy and adultery after marrying a Christian man.
She was sentenced to be flogged and hanged.
Ms Ibrahim gave birth to her second child while in prison.
Sudan had come under intense international pressure to have her released.