Dozens of new Ebola cases have erupted in Liberia, near the border with Sierra Leone, Liberian health officials warned Monday, marking a setback amid recent improvements.

It also threatens the country's goal of recording no new cases of the disease by the end of the year.

The new cases bring the number of people infected by Ebola in the three countries worst affected by the outbreak to 20,081, the World Health Organization said on Monday. Among them, more than 7,842 deaths have been reported.

The flare-up is due to a number of factors including people going in and out of Liberia and traditional practices such as the washing of bodies, said Liberia's Assistant Health Minister Tolbert Nyenswah.

Forty-nine cases were reported in in western Grand Cape Mount County between December 1 and 25, Nyenswah told state radio. This included 27 confirmed cases, nine probable and 13 suspected, said Nyensuwah.

The government had set a Dec. 31 target for recording no new Ebola infections. The Grand Cape Mount outbreak makes hitting that target unlikely.

"In a very small population, an increase in the number of [Ebola] cases raises high level of concerns that we need to take very seriously as people of Liberia and people of Grand Cape Mount in particular," he said.

Grand Cape Mount is one of Liberia's least populous counties, with just over 140,000 people. It borders Sierra Leone, where the deadly hemorrhagic fever is also raging.

Nearly 3,400 people have died from Ebola in Liberia over the past year with nearly 8,000 cases total, though health officials say the situation has improved, especially in the capital, Monrovia.

Sierra Leone, in comparison, has now eclipsed Liberia with more than 9,000 Ebola cases, according to the World Health Organization.