(Recasts, adds quotes, background) By Richard Waddington GENEVA, March 13 (Reuters) - Sudan on Tuesday dismissed as invalid a United Nations human rights mission which accused it of orchestrating gross violations in Darfur and declared that the humanitarian situation there had improved. Signalling the start of a diplomatic tussle at the U.N. Human Rights Council, which dispatched the mission, Sudan accused the team's leader of bias and said the mission should not have gone ahead after some members dropped out. "We therefore strongly and resolutely oppose any consideration by this esteemed Council of any report that comes out of this mission," Sudan's Justice Minister Mohamed Ali Elmardi told the 47-state Council. The team, led by Nobel Peace Prize laureate Jody Williams, was asked to investigate charges of widespread abuse in Darfur, where observers say some 200,000 people have been killed and more than 2 million driven from their homes since a revolt broke out in 2003. Reporting on Monday, the team said Darfur was "characterised by gross and systematic violations of human rights". It declared that the government had "manifestly failed to protect the population ... from large-scale international crimes and has itself orchestrated and participated in these crimes". Sudan denies responsibility for abuses, which Washington calls genocide, and blames them on rebel groups which refused a 2006 peace deal. It also says the death toll is exaggerated and that Western media have blown the conflict out of proportion. Asked to comment on the findings, U.N. High Commissioner for Human Rights Louise Arbour called it "very authoritative" and said it added weight to growing evidence of "massive crimes against humanity" in the vast desert region bordering Chad. 'PRECONCEIVED AND HOSTILE' But diplomats said there was a struggle within the Council, formed last year to give greater credibility to U.N. efforts to protect human rights, over the handling of the findings, with some Muslim states wanting a new mission to be sent. In his speech, the Sudanese minister said two of the five-member team appointed by the chairman of the Council to report on Darfur had dropped out of the mission, leaving it without legitimacy. But only Indonesia's ambassador officially withdrew after Khartoum refused to grant visas to team members. Gabon's ambassador did not travel with the others to Chad, where the team interviewed humanitarian officials and refugees. But he signed up to the report's conclusions, which are due to be presented formally to the Council on Friday. Although it gave no official reason for refusing the visas, Sudan had objected to the inclusion of Bertrand Ramcharan, a Guyanan who was the U.N.'s acting human rights chief in 2003-2004 and sent the world body's first rights team to Darfur. Elmardi also accused Williams, an anti-landmine campaigner, of having a "preconceived and hostile attitude against Sudan". Since the peace deal with one rebel group, there had been tremendous changes for the better in Darfur, the minister said. "The humanitarian situation is much more stable now and there is visible decrease in malnutrition and mortality rates," Elmardi said. Humanitarian agencies, including those of the U.N., say however that they face immense difficulties in getting assistance to the needy in Darfur, an area the size of France. (Additional reporting by Anne Richardson in Geneva)