MONROVIA (Reuters) - Liberia’s President Ellen Johnson Sirleaf has sacked 10 senior officials because they failed to heed a warning to return from overseas travel to help the government’s fight against an Ebola epidemic that has killed at least 1,100 Liberians.

Liberian President Ellen Johnson-Sirleaf attends the opening ceremony of the 22nd Ordinary Session of the African Union summit in Ethiopia's capital Addis Ababa, January 30, 2014. REUTERS/Tiksa Negeri/Files

The officials, who include six assistant ministers, two deputy ministers and two commissioners, were dismissed with immediate effect for being “out of the country without an excuse,” according to a statement from the president’s office.

They were initially told in August to return to Liberia.

“These government officials showed insensitivity to our national tragedy and disregard for authority,” said the statement released late on Saturday. It did not make clear what role the government expected the officials to play in the response to the crisis, or why they were out of the country.

The contagious, haemorrhagic fever was first discovered in eastern Guinea in March and has killed more than 2,400 people, mostly in Liberia, Sierra Leone and Guinea, making it the worst Ebola outbreak the world has seen.

In the process, it has stretched the understaffed and poorly resourced healthcare systems of those countries to breaking point.

The World Health Organization (WHO) has warned that the epidemic is spreading exponentially in Liberia, where more than half of the deaths have been recorded. It has said that thousands are at risk of contagion in the coming weeks.

Sirleaf on Saturday appealed to U.S. President Barack Obama for urgent aid in tackling Ebola.

The disease has taken a particularly heavy toll on healthcare workers who have stationed themselves on the frontline of the fight against the disease.

Some 144 healthcare workers have died in Guinea, Sierra Leone and Liberia, according to Sept. 7 figures from the World Health Organisation (WHO).The first Sierra Leonean female doctor to be diagnosed with Ebola died on Sunday, according to two government sources.

Olivette Buck was head of the Lumley Health Centre in a densely-populated suburb west of the capital Freetown. She tested positive for the virus on Tuesday, apparently contracting it as she treated an Ebola patient.

“I can confirm that doctor Olivette Buck died between last night and this morning,” Jarrah Kawusu-Konteh, of the State House communication unit, told Reuters.

Doctors are held in high esteem in countries like Sierra Leone that have a low percentage of trained medical professionals per head of population. She was the fourth Sierra Leonean doctor to die of Ebola.

Her death came amid calls for her evacuation to Germany for treatment. Civil society group WeCare SL and the Sierra Leone Medical and Dental Association both urged the government to save her.

President Ernest Bai Koroma also wrote to the WHO on Friday requesting the U.N. health agency to evacuate Buck, according to a letter seen by Reuters.

A senior government official said the WHO declined the request, but offered instead to make available an experimental treatment locally. There was no immediate response from the WHO to a request for comment.