A medical aid agency says increasing insecurity has forced it to stop work in a volatile part of southern Sudan.

Doctors Without Borders (Medecins Sans Frontieres) said Friday gunmen robbed its staff and clinic in Gumuruk, in Jonglei state, three separate times in July.

Twice, armed groups stole therapeutic ready-to-use food intended for malnourished children, and on the second occasion they also stole medical equipment.

And in a third incident on July 27, three staff members were violently robbed at gunpoint.

The head of the group in southern Sudan, Rob Mulder, called the incidents "unacceptable," saying they prevent "essential medical aid."

More than 160 malnourished children were receiving treatment and food on an ongoing basis, and Doctors Without Borders said more than 20 new patients appeared each week.

Additionally, the clinic provided basic services to the community and emergency interventions for pregnant women and children with cerebral malaria or severe anemia.

Unrest has been on the rise in southern Sudan in the past year.  For more than two decades, southern militants fought a brutal civil war against the government in the north. That war ended in 2005, but groups in the region remain heavily armed and in competition for scarce resources.

Southern leaders have also accused Sudan's ruling National Congress Party of trying to stir up trouble in the region ahead of a referendum scheduled for January.

In that vote, southern Sudanese will decide whether to become an independent state. The referendum is part of the peace deal that ended the civil war.

Sudanese President Omar al-Bashir has called for Sudan to remain unified, though he has said he will accept the south's decision.

Some information for this report was provided by AP.