BBC News with Jerry Smit.

The United Nations Humanitarian Co-coordinator in South Sudan Toby Lanzer has told the BBC that hundreds of thousands of people are fearing for their lives amid escalating ethnic violence. Fighting broke out a weeks ago between soldiers loyal to President Salva Kiir and those supporting his former deputy Riek Machar. Here's our Africa editor Richard Hamilton.

Mr. Lanzer said there was an atmosphere of fear and desperation as well as a cycle of revenge that has gone to extremes. He said the speed with which the situation had unraveled was astonishing. Just over two weeks ago, he said, an investment conference has been held in the capital Juba. But now people were being lined up and executed on the streets. He wasn't sure where the country would be in one week's time, but he feared dark clouds were gathering over South Sudan, bringing with it a humanitarian storm.

The newly released former Russian oil tycoon Mikhail Khodorkovsky says he will do all he can to help political prisoners but will not seek political power. He spent ten years in a Russian prison for fraud and tax evasion, charges he said were political motivated. Mr. Khodorkovsky spoke to reporters in Berlin, Daniel Stanford was there.

The Berlin news conference was the first time Mikhail Khorkovsky has been seen in public as a free man in more than ten years. It was widely assumed that it was Vladimir Putin who ensured that he stayed in jail. But on Thursday, President Putin pardoned him. Mikhail Khorkovsky said he would not be returning to Russia because he had no guarantees that he'd be able to leave again. His imprisonment continues a long tradition in Russia politics of dissidents being silenced, going back to the Decembrists in 1825 and Dostoevsky.

A court in the Egyptian capital Cairo has sentenced three prominent pro-democracy activists to three years each in prison with hard labor for their roles in recent protests. The men are the first secular figures to be convicted under a new protest law which human rights group says to stifle public protests. The three are leading figures in the protest movements that triggered the 2011 uprising that put an end to President Hosni Mubarak's 29-year rule.

The British Prime Minister David Cameron says the Syrian government should answer for the death of a British doctor Abbas Khan while in detention in Syria. The surgeon went there last year but he was arrested shortly after his arrival and found dead just days before he was due to be released. His body has now been brought back to London. His sister Sara Khan said the family were determined to find out how he died.

The body's been taken today to the hospital for a CT scan. And we have a pathologist which the Home Office instructed and we've also got our own independent pathologist to undergo a postmortem. And we want it to be a non-invasive postmortem; we want as many answers as possible.

World News from the BBC.

Reports from the Central African Republic say that thousands of Muslims have staged a demonstration in the capital Bangui against French troops who are conducting a disarmament operation. France sent 1,600 troops to the CAR earlier this month to support an African Union force which has struggled to cope with an upsurge of Christian Muslim violence.

Ten people in the Kenyan capital Nairobi have been injured after a freight train derailed in a slum area of the city. Four cargo wagons toppled onto a number of flimsy trackside homes in the Kibera's slum. Rescuers tried to clear space in the crowded settlements to bring in cranes to reach anyone who may have been trapped. However the Red Cross said they had no reports of any missing people so far.

An American teenager who was shot in the head by a fellow pupil last week has died. The 17-year-old victim Claire Davis has been in coma since she was promptly randomly targeted at her school in Denver. The 18-year-old gunman Karl Pierson had entered the school carrying a shot gun, machete and three incendiary devices. He later killed himself.

Researchers say they have discovered a vast reservoir of melt water beneath the Greenland ice sheet that remains liquid even in winter. The scientists say the water could yield clues about rising sea levels. Our environment correspondent Matt McGrath reports.

The melting of the Greenland ice sheet increased rapidly over the past decade, but scientists are still uncertain about the ultimate destination of the outflows of water. Now researchers say they've detected a large quantity of liquid stored in partially compacted snow. This massive aquifer covers 70,000 sq km, and the researchers believe it contains about 140 billion tons of water enough to raise global sea levels by 0.4 millimeters. But the researchers are still uncertain that this new discovery is leaching into the sea or not.Matt McGrath, BBC News.