UNITED NATIONS (Reuters) - Sudan's government has carried out at least 30 likely chemical weapons attacks in the Jebel Marra area of Darfur since January using what two experts concluded was a probable blister agent, Amnesty International said on Thursday.

The rights group estimated that up to 250 people may have died as a result of exposure to the chemical weapons agents.

The most recent attack occurred on Sept. 9 and Amnesty said its investigation was based on satellite imagery, more than 200 interviews and expert analysis of images showing injuries.

"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.

Sudanese U.N. 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.

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.

Sudan joined the Chemical Weapons Convention in 1999 under which members agree to never use toxic arms.

The Organization for the Prohibition of Chemical Weapons in the Hague, which oversees adherence to the treaty, said in a reaction it would examine the Amnesty report "and all other available relevant information."

The organization's executive council is due to meet Oct. 11-14.

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.

Some 300,000 people have been killed in Darfur since the conflict began in 2003, the U.N. says, 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.

(Reporting by Michelle Nichols; Editing by Tom Brown)