By Elizabeth Barber, TIME

The U.N. mission responsible for responding to the Ebola outbreak will miss its Dec. 1 target for containing the disease because of rising transmission rates in the West African countries of Sierra Leone and Mali.

Anthony Banbury, the head of the U.N. Mission for Ebola Emergency Response (UNMEER), told Reuters that though progress has been made in some areas — including in Liberia, one of the countries hardest hit by the current outbreak — setbacks elsewhere have put the mission off its target.

UNMEER said in September that it hoped to have 70% of Ebola patients in treatment, and 70% of Ebola victims safely buried, by the start of next month. But just 13% of Ebola patients have been isolated in Sierra Leone, according to a UNMEER statement.

“Progress is slow and we are falling short, and we need to accelerate our efforts,” said Amadu Kamara, the U.N.’s Ebola crisis manager for Sierra Leone, in a statement.

The Ebola virus has killed some 5,459 people worldwide, mostly in Liberia, Sierra Leone and Guinea. The most recent World Health Organization situation report, released on Friday, describes transmission rates in all three countries as “intense.”

Mali, which was believed to be Ebola-free after a toddler’s death from the virus there in October, said on Monday that an eighth person in the nation had tested positive for the disease.

Still, UNMEER said that it is hopeful that efforts to stop the virus in Mali will benefit from lessons learned in the three nations still reeling from Ebola.

Banbury also told Reuters that Liberia was a bright point in the mission’s efforts to contain the virus.

Liberia’s President expressed optimism at a ceremony on Monday that her country, whose economy has been gutted by the outbreak, could still reach its goal of no new Ebola cases by Christmas.

“We’ve set a pretty tough target,” said President Ellen Johnson Sirleaf, the Associated Press reports. “But when you set a target, it means that you stay focused on that target and on that goal.”