By Katy Migiro

NAIROBI, Nov 19 (Thomson Reuters Foundation) - Some 48,000 displaced South Sudanese sheltering in a United Nations base are falling ill in large numbers because of overcrowded and substandard living conditions, the medical charity Medecins Sans Frontieres said on Thursday.

The MSF hospital in the U.N. base in Malakal, capital of northern Upper Nile State, has been filled beyond capacity in recent weeks with children suffering life-threatening diseases like pneumonia and malaria, it said.

The camp's population has doubled this year, mostly through the arrival of women and children, and there has been a fivefold increase since July in the number of under-five-year-olds being treated in the hospital.

"The sickness of our patients is directly related to the overcrowded and deplorable conditions in which they are living," MSF's programme manager for South Sudan, Monica Camacho, said in a statement.

"More space must immediately be allocated to the people seeking shelter, and aid organisations must urgently improve the provision of basic services."

People have been sheltering in the base since conflict erupted in South Sudan in December 2013, when clashes broke out in the capital Juba between troops loyal to President Salva Kiir and soldiers backing Riek Machar, the former vice president.

Kiir and Machar, facing international pressure and the threat of sanctions, signed a peace deal in August but have repeatedly accused each other of ceasefire violations.

The civil war has displaced more than 2.2 million people, mostly in the northern oil-producing Unity, Jonglei and Upper Nile states, where hunger is most acute.

Humanitarians have said malnutrition is at catastrophic levels and there is a risk of famine in some places towards the end of the year.

Some 7,000 new arrivals, mostly women and children, are living in marshy parts of the camp, where children play in mud surrounded by rubbish, MSF said.

Over 50 people share each communal tent, often without blankets. Each has less than 4.5 square metres of living space, far less than the 30 square metres required by international humanitarian standards, the agency said.

Water shortages and overcrowding across the camp are contributing to poor health, it said, with one latrine per 70 people, less than one-third of the ratio required by agreed humanitarian standards.

The onset of the rainy season has made pneumonia a problem, given the crowded, unhygienic conditions, MSF said. The number of patients treated for severe respiratory tract infections has tripled since September. (Reporting by Katy Migiro, editing by Tim Pearce. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, corruption and climate change)