Liberia's President Ellen Johnson Sirleaf has sacked ten 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.

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 back in August to return to Liberia.

"These government ­officials showed insensitivity to our national tragedy and disregard for authority," said the statement.

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 Organisation (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 US 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 figures from the 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.