Italian energy company Eni raised the death toll by one to 13 after last week's explosion during repairs on an oil pipeline in Nigeria's Niger Delta region.

Eni reported an explosion Thursday on the Tebidaba-Clough Creek oil pipeline. The explosion on the pipeline came as workers were trying to repair damage the company blamed on saboteurs.

The death toll from the explosion was raised Saturday after one of three people injured in the incident died at a hospital. In total, the explosion left 13 people dead and two injured.

"We reiterate our determination to shed full light on the matter, in close cooperation with local authorities," Eni Chief Executive Officer Claudio Descalzi said in a statement.

The company in 2013 blamed criminal theft and sabotage for the loss off roughly 30,000 barrels of Nigerian oil per day, leading to losses of approximately $554 million. In its latest update, the company said it "appears likely" the fire was caused by parties behind crude oil theft from the Tebidaba-Clough Creek pipeline.

"Some local witnesses have reported seeing saboteurs, upon arrival of the intervention team to repair the pipeline, at the point of theft of the oil where the same saboteurs had installed a specific valve," the company said.

Amnesty International has expressed frustration with Eni and its counterparts at Royal Dutch Shell over the legacy of oil spills in the Niger Delta region. The rights group said the two companies combined reported more than 550 spills in the area last year.