The Brussels-based branch of international medical charity MSF has ended its operations in Sudan, saying its work had been made impossible by the authorities in Khartoum.
The charity had been giving medical aid to displaced people in East Darfur, South Darfur and Blue Nile State.
But it said it had been "systematically blocked" from continuing the work.
It accused Khartoum of prioritising military operations over humanitarian assistance to people in desperate need.
Sudan has been fighting a rebellion in Darfur since 2003 and separate insurgencies in Blue Nile and South Kordofan state since South Sudan seceded in 2011.
MSF said it had experienced "total denial of access" to Blue Nile State, "forced closure of activities" in East Darfur and "administrative obstacles and blockages" in South Darfur.
Dr Bart Janssens, MSF operations director in Brussels, told the BBC's Focus on Africa programme that it had been a "very difficult decision" to pull out.
He said the areas where MSF had been able to work in Sudan had been "constantly reduced" by the Sudanese government over the past three years.
In a statement, Dr Janssens said: "Whether we ask for access through dialogue and meetings, seek negotiations through influential partners to the government, or speak out in media, nothing seems to have the slightest impact.
"Our experience is that the Sudanese government arranges meetings specifically to prevent international aid, rather than to facilitate it.
BBC Sudan analyst James Copnall says MSF is saying in public what many aid groups operating in Sudan say in private.
The Sudanese government has a reputation for obstructing aid, in particular into areas it considers support rebel movements or dislike the government, he says.
Other sections of MSF are continuing to work in other parts of Sudan.
However, nine days ago, the charity's French branch pulled out of South Kordofan after its hospital in Frandala was bombed by a Sudanese government warplane.
Rebels in the state formerly allied with South Sudanese groups took up arms against the Khartoum government in 2011.