An aid official has been detained in Sudan's Darfur region, a day after his director was charged with spying and spreading false information.
Some two million people have fled their homes in Darfur
Vince Hoedt, Darfur co-ordinator for the Dutch section of Medecins Sans Frontieres has not yet been charged.
MSF Sudan director Paul Foreman was arrested on Monday and later released on bail, over a report on rape.
BBC Africa analyst Martin Plaut says the charges are part of a drive by Sudan to end western criticism.
The Sudanese authorities deny accusations that they back the Arab Janjaweed militias alleged to have committed widespread atrocities, such as mass killings and mass rape.
They also deny that the scale of the violence is as severe as reported by aid agencies.
The BBC's Martin Plaut, who recently travelled to Darfur, says that many Sudanese believe western aid workers have given information on alleged human rights abuses in Darfur to the United Nations, which has passed a sealed list of 51 war crimes suspects to the International Criminal Court.
Our correspondent says that in March, aid workers were threatened over their reports of mass rape.
Foreman was arrested on Monday
Mr Hoedt was arrested in Darfur and flown to the capital, Khartoum, under police escort.
United Nations Sudan envoy Jan Pronk has condemned the "smear campaign" in Sudanese newspapers against aid workers, accusing them of fabricating reports of rape.
"MSF Holland and all the other MSF have saved many lives of Sudanese people," he said.
The state crime prosecutor said Mr Foreman had failed to hand over evidence on which the report on rape was based.
Mr Foreman said "medical privilege" and patient confidentiality prevented him from handing over documents requested by the authorities.
MSF Holland spokesman Geoff Prescott said another reason for respecting the information, was because women "made pregnant as a result of rape outside wedlock can be arrested by the authorities" in Sudan, which operates strict Islamic sharia law.
He told the BBC's Focus on Africa programme that the charity stood by its report, which he described as "accurate and truthful".
Sudan's state crime prosecutor said he had come to conclusion that the report was false.
Mr Foreman could face up to three years in prison if found guilty of falsifying the report.
It is not yet known when he will appear in court.
"We would like to reiterate that we think it's the people who perpetrate rape in Darfur who should be in court, not the people who are trying to bring medical assistance to the victims," Mr Prescott said.
The report - The Crushing Burden of Rape: Sexual Violence in Darfur - which came out in March, was based on the treatment of 500 women over a four-and-a-half month period in Darfur.
It details nearly 300 of these cases, with several written up as witness statements, Mr Foreman said.
Human Rights Watch Africa director Peter Takirambudde said Mr Foreman's arrest was "a perfect illustration of how far the Sudanese government is prepared to go to silence criticism and deny its own responsibility for massive atrocities in Darfur."
MSF says it has a significant presence in Darfur, with some 180 international staff and 3,000 local staff treating some one million patients.
The UN says that about 180,000 people have died in the two-year conflict in Darfur, and more than two million driven from their homes.