KINSHASA (Reuters) - An outbreak of Ebola has killed one person and is believed to have infected three others over the last week in northeastern Congo, medical charity Medecins Sans Frontieres said on Friday.

The outbreak is in Isiro, a busy town in Democratic Republic of Congo's Oriental province, which shares a border with Uganda, but the strain of the deadly disease is different to the one that killed 16 there last month, MSF said.

Ebola is transmitted to humans from monkeys and birds and causes massive bleeding in victims, with mortality rates as high as 90 percent.

Anja de Weggheleire, the medical coordinator for MSF in the area, said blood samples from one victim had confirmed Ebola in Isiro and there were at least three other suspected cases being treated in an MSF-supported local hospital.

"We cannot speak of a direct link between the two epidemics, I think unfortunately it's just pure coincidence," de Weggheleire told Reuters.

MSF was helping track and isolate people who may have come in contact with the disease, she added.

Authorities in Uganda said this week that the outbreak there was under control after they imposed strict measures to prevent Ebola from spreading in the west of the country.

However, Congo's health system is permanently stretched and MSF warned that preventing the spread of the disease from the town, a provincial transit point, could be a challenge.

"(The situation) is quite serious already ... Isiro is quite a busy place, quite well connected, that could make it quite complex to contain (the fever)," de Weggheleire added.

(Reporting by Jonny Hogg; Editing by David Lewis)