Sudanese government forces may have used chemical weapons against civilians in the western region of Darfur in the past eight months, killing as many as 250 people, Amnesty International said.

The London-based advocacy group said Thursday it has gathered “credible evidence” of at least 30 likely chemical attacks in Darfur’s Jebel Marra region between January and September. Many of the people that may have died due to exposure to the weapons were children, it said in a statement, citing testimonies from caregivers and survivors. Sudanese Information Minister Ahmed Bilal Osman didn’t answer three phone calls seeking comment.

“The scale and brutality of these attacks is hard to put into words,” said Tirana Hassan, Amnesty’s director of crisis research. Two independent chemical-weapons experts consulted by Amnesty said the findings “strongly suggested exposure to vesicants, or blister agents, such as the chemical warfare agents sulfur mustard, lewisite or nitrogen mustard,” the group said.

The suspected chemical assaults came during a large-scale military offensive that began in January against the Sudan Liberation Army/Abdul Wahid faction, rebels whom Sudan’s government say have ambushed army convoys and attacked civilians. Conflict that began in Darfur in 2003, when insurgents took up arms accusing President Omar al-Bashir of neglecting the region, has led to the deaths of about 300,000 people and forced 2.5 million from their homes, according to United Nations estimates. Mr. al-Bashir has been indicted by the International Criminal Court for alleged war crimes.

The chemical-weapons agents were said to be in bombs dropped from planes and rockets, according to Amnesty. Hundreds who survived the attacks later developed symptoms including severe gastrointestinal conditions, blistering and rashes, loss of vision and respiratory problems, the group said.

Amnesty also cited satellite imagery showing that 171 villages were destroyed or damaged between January and September, the “overwhelming majority” of which, it said, had no formal armed opposition presence. It called on the UN Security Council to urgently investigate the alleged chemical attacks.

“This suspected use of chemical weapons represents not only a new low in the catalog of crimes under international law by the Sudanese military against civilians in Darfur, but also a new level of hubris by the government towards the international community,” Ms. Hassan said.