Things have rapidly deteriorated in parts of South Sudan since political infighting between President Salva Kiir and ex-Vice President Riek Machar left Juba locked down on December 15th. A country that was described as “teetering on the brink” a week ago now looks like all-out civil war.

While this is a political crisis first and foremost, it is playing out along ethnic lines, and to very frightening effect. Human Rights Watch has reported that both soldiers and rebels have been seen executing people based on their ethnicity (Dinka or Nuer). Daniel Howden describes what can only be labeled a massacre in Juba (and several others like it) at The Guardian. The most horrifying example of what’s now happening there:

A week ago, Simon K, a 20-year-old student living in the capital of South Sudan, was arrested by men in military uniforms. He was asked a question that has taken on deadly importance in the world’s newest country in the past seven days: incholdi – “What is your name?” in Dinka, the language of the country’s president and its largest ethnic group.

Those who, like Simon, were unable to answer, risked being identified as Nuer, the ethnic group of the former vice-president now leading the armed opposition and facing the brunt of what insiders are describing as the world’s newest civil war.

Simon K was taken to a police station in the Gudele market district of Juba, where he was marched past several dead bodies and locked in a room with other young men, all Nuer. “We counted ourselves and found we were 252,” he told the Guardian. “Then they put guns in through the windows and started to shoot us.”

The massacre continued for two days with soldiers returning at intervals to shoot again if they saw any sign of life. Simon was one of 12 men to survive the assault by covering themselves in the bodies of the dead and dying.

Outside of Juba, the reverse is happening. Armed groups have taken much of Jonglei and Unity states, including Bor and Bentiu, their respective capitols. The UN humanitarian coordinator said that he saw “people who were being lined up and executed in a summary fashion” in Bor. Tens of thousands have sought refuge at UN compounds, and expatriates from many countries have already been evacuated.

As we hold our breath and hope that calm can be restored, talks can be mediated, and people can safely return home, I’ll try to keep you updated. In the meantime, a good primer is Radio Tamazuj’s nine questions about the South Sudan crisis and Think Africa Press’ edition of experts weekly focusing on the crisis. If you’re feeling like historicizing, this HRW dispatch on ethnic tension from earlier this year and this FP piece on how South Sudan faced setbacks from the beginning. If you’re on Twitter, Lesley Warner has compiled a long list of South Sudanese Twitter handles you can follow to get news, and the bottom of this post also has some expats that were (might still be) tweeting from Juba.

Update: Colum Lynch just wrote a good piece outlining how things have devolved.