This is extremely worrying. The ethnic violence that has broken out in South Sudan involves a deadly rivalry between two tribes and it is believed that the humanitarian workers may have been caught up in the conflict.

CBS News:

The latest news from South Sudan is especially worrying. Some 130 staff at the Doctors Without Borders clinic in Pibor, South Sudan, are missing, according to a statement from the group.

Renewed ethnic violence in Jonglei State, South Sudan, has forced thousands of families to flee into the bush. Two Doctors Without Borders medical facilities have been targeted and the independent medical humanitarian organization has been forced to temporarily suspend its much needed medical activities in Pibor County.

Doctors and nurses had to evacuate a hospital in the town of Pibor and two outreach clinics as a column of 6,000 armed youths from the Lou Nuer tribe descended on the town in pursuit of a rival tribe, the Murle, according to the Telegraph. The hospital was reportedly torched by the attacking forces.

The Lou Nuer and Murle are in conflict, with each accusing the other of stealing cattle and killing tribal members. Neither UN peacekeepers nor the South Sudan army were able to stop the attack on Pibor.

"Thousands of people have fled for their lives in Lekongole and Pibor in the last week and are now hiding in the bush, frightened for their lives," said Parthesarathy Rajendran, DWB head of mission in South Sudan. "They fled in haste and have no food or water, some of them doubtless carrying wounds or injuries, and now they are on their own, hiding, beyond the reach of humanitarian assistance."

An estimated 150 people were killed, according to the United Nations, but now the invading band has gone back to their home areas, according to the BBC. However, the thousands who fled the area, including the medical staff, are still missing and believed to be hiding the the bush.

Hopefully, the staff escaped into the bush and were able to avoid the bloodletting. But despite the end of the violence, no word yet on the fate of most of those workers, although DWB have issued a statement saying they have been in touch with some of the 156 workers who took flight.