Sudan's government has been accused of killing scores of civilians this year in at least 30 suspected chemical weapons attacks in a remote area of the country's Darfur region.
In a report released on Thursday, Amnesty International estimated that up to 250 people, including many children, may have died as a result of exposure to the chemical weapons agents in the Jebel Marra area since January.
The UK-based rights group, which said that its investigation was based on satellite imagery and more than 200 interviews, alleged that the most recent attack occurred on September 9.
"The use of chemical weapons is a war crime. The evidence we have gathered is credible and portrays a regime that is intent on directing attacks against the civilian population in Darfur without any fear of international retribution," said Tirana Hassan, Amnesty International's director of Crisis Research.
Amnesty said it had presented its findings to two independent chemical weapons experts.
"Both concluded that the evidence strongly suggested exposure to vesicants, or blister agents, such as the chemical warfare agents sulfur mustard, lewisite or nitrogen mustard," Amnesty said in a statement.
READ MORE: UN warns of worsening situation in Sudan's Darfur
Sudanese UN Ambassador Omer Dahab Fadl Mohamed said in a statement that the Amnesty report was "utterly unfounded" and that Sudan does not possess any type of chemical weapons.
"The allegations of use of chemical weapons by Sudanese Armed Forces is baseless and fabricated. The ultimate objective of such wild accusation, is to steer confusion in the on-going processes aimed at deepening peace and stability and enhancing economic development and social cohesion in Sudan," he said.
Sudan joined the Chemical Weapons Convention in 1999 under which members agree to never use toxic arms.
A joint African Union-United Nations force, known as UNAMID, has been stationed in Darfur since 2007. Security remains fragile in Darfur, where mainly non-Arab tribes have been fighting the Arab-led government in Khartoum, and the government is struggling to control rural areas.
Darfur has been wracked by conflict since 2003 when ethnic groups rebelled against the government.
According to the UN, at least 300,000 people have been killed since the fighting began, while 4.4 million people need aid and over 2.5 million have been displaced.
The International Criminal Court issued arrest warrants for Sudanese President Omar Hassan al-Bashir in 2009 and 2010 on charges of war crimes and genocide in his drive to crush the Darfur revolt.