(CNN) The Sudanese government has been accused of using chemical weapons against the people of Darfur, according to a report released by Amnesty International on Thursday.

The attacks targeted civilians and may constitute a war crime, the report said. The Sudanese government has denied the allegations, calling them "rumors."

Amnesty says it has new evidence abuses persist in a war that has been described as one of the world's worst humanitarian conflicts by the United Nations.

Unidentified witnesses quoted in the Amnesty report said the attacks left a smoke, which turned dark blue and smelled "like rotten eggs," coating the trees, ground, and humans in a thick black dust.

Witness testimony

After exposure to the smoke, some said their skin turned white and became rotten or hardened and fell off in chunks.

"When the bomb exploded I inhaled the poisonous air which I am still smelling even now," said one witness.

Some children vomited blood, the report said. Another witness said: "My youngest child was walking before the attack. Now she is only crawling."

Pictures obtained by Amnesty International show graphic images of children with large welts, peeling skin, and infected lesions.

Civilians targeted

The alleged use of chemical weapons came during a large-scale offensive by the Sudanese forces and its allied groups against an armed opposition group, the Sudan Liberation Army-Abdul Wahid (SLA-AW), which operates in the Jebel Marra region.

The Sudanese government accused the group of looting and attacking civilians and military vehicles prior to the January offensive.

Since the Sudanese military campaign began in January, Amnesty International says up to 250 people have been killed by chemical weapons.

The report alleges the Sudanese forces targeted civilians: "The overwhelming majority of the attacked villages had no formal armed opposition presence at the time of the attacks. The purpose...appears to have been to target the entire population of the village."

Chemical attacks in some regions have been taking place for eight months, including just weeks before the report's release.

'Rumors'

The Sudanese government has denied allegations it had used chemical weapons against civilians, calling them "rumors."

"I don't know from where these rumors are being said," Sudan's Information Minister Ahmed Bilal told CNN.

Bilal acknowledged a government offensive in Jebel Marra had taken place, saying it was in response to rebel activity.

"It was started by them," Bilal said. "The rebels were doing some sort of looting, they were attacking innocent people. This has stopped. There is not an inch occupied by rebels," Bilal said.

"The whole Darfur is quite in peace and the people are very happy," he said.

Likely more than one chemical

Journalists and humanitarians have been prohibited from entering Jebel Marra for more than four years, making reporting extremely difficult from the region.

Mustard agent Amnesty International claims to have evidence of the use of suspected chemical weapons in Darfur. It says two independent experts have concluded the evidence suggests exposure to blister agents. Blister agents include sulfur mustard and nitrogen mustard. Sulfur mustard has long been in use as a chemical weapon -- it inflicted horrific casualties in World War I.

Mustard agents inflict chemical burns on the skin, eyes and lungs, and can also affect the internal organs. Victims can be disabled as a result of exposure. It can be fatal if they come into contact with large amounts.

While damage is caused to the body's cells within minutes of contact with mustard agent, it can take hours before the full effects are felt.

Symptoms can include blistering, itching or severe burning of the skin; weeping eyes, swollen eyelids or even blindness; vomiting and collapse.

The effects of severe exposure can last for years.

There's no antidote for mustard agent injury. Sources: U.S. Centers for Disease Control and Prevention; Organisation for the Prohibition of Chemical Weapons

Amnesty International had to do all reporting remotely. Collecting soil samples for confirmation was impossible.

Two chemical weapons experts reviewed photographic and video evidence and both found the symptoms consistent with chemical agents such as sulfur mustard, lewisite and nitrogen mustard -- or a combination.

"The symptoms varied between the attacks and this tells me there were likely more than one chemical in use as well as the possibility that the chemicals were mixed or that different chemicals were used at different times for different attacks," said Dr. Jennifer Knaack, one of the weapons experts involved in the study.

The writer of the report, a senior Amnesty adviser, Jonathan Loeb calls it "by far the most substantial, credible release of evidence of the use of chemical weapons in Darfur since the conflict began."

History of conflict

The conflict in Darfur began around 2003 when several rebel groups in Darfur took up arms against the government in Khartoum. They had grievances over land and historical marginalization.

In response, the government's counterinsurgency strategy targeted the opposition groups but reportedly expanded to target tribes associated with the insurgents.

The violence escalated into a war and the in 2008, the UN estimated that 300,000 people may have died in the Darfur conflict, although experts say that figure has likely risen since then.

Sudan's President, Omar al-Bashir, was charged with crimes against humanity by the International Criminal Court, including genocide, related to the Darfur conflict in 2010.

Bashir has yet to cooperate with the court and continues to travel freely around the continent. South Africa and Uganda have both been criticized for allowing President Bashir to travel to their countries without being turned over to the ICC.