A pregnant teenager who was gang-raped and ignored when she tried to report the crime has been convicted of "indecent acts" by a court in Sudan.

The victim, an Ethiopian migrant, was sentenced to one month in prison, which has been suspended, and fined 5,000 Sudanese pounds (£528). The verdict was condemned by activists who said it would discourage rape victims from speaking out and entrench "a culture of impunity" for perpetrators.

The 18-year-old victim was searching for a new home when she was lured to an empty property in the capital, Khartoum, attacked by seven men and gang-raped. The incident was filmed by the perpetrators and distributed through social media six months later, triggering the arrests of everyone involved.

The woman, who is nine months pregnant, was initially charged with adultery and faced a possible sentence of death by stoning. This was dropped when the court accepted she is divorced. Since her arrest she has been detained in police cells and her requests for a transfer to medical facilities have been refused, the Strategic Initiative for Women in the Horn of Africa (SIHA) network said.

Of the seven men on trial, three were convicted of adultery and sentenced to 100 lashes and two were convicted of indecent acts and sentenced to 40 lashes and fines. One convicted of distributing indecent material was sentenced to 40 lashes and fined. A seventh man was freed owing to insufficient evidence against him.

The SIHA said those subject to lashings had their sentences carried out immediately in a closed court setting. The woman has been threatened by the court with further punishment for entering the country illegally, it added.

The SIHA condemned Thursday's verdict. Hala Elkarib, its regional director, said: "This verdict reflects the substantial challenges in enabling victims of sexual violence to pursue justice. It will also serve to prevent future victims from speaking out and seeking assistance and entrenches a culture of impunity for perpetrators.

"Women migrants and IDPs [internally displaced persons] are some of the most marginalised people in Sudan and most vulnerable to violence, abuse and persecution. The Sudanese judiciary today has demonstrated its incapacity to protect the most vulnerable in society and instead attempt to delegitimise those that experience abuse at the hands of its citizens."

She added: "The levelling of immigration charges against the victim further denies her protection by the state and protracts the punishment and emotional stress against her whilst she has been subjected to the most brutal of crimes."

Although rarely carried out, the sentence of stoning for adultery has been handed down twice in recent years, against two women, Intisar Sharif and Laila Jamool, in 2012. Following appeals in both cases, the sentences were overturned.

Last year a Somali woman who alleged she was raped was sentenced to a year behind bars. She was eventually acquitted after international protests.