MONROVIA, August 27 - Liberia's leader has sacked ministers and senior government officials who defied an order to return to the west African nation to lead the fight against the deadly Ebola outbreak, her office said on Tuesday. President Ellen Johnson Sirleaf had told overseas ministers to return within a week as part of a state-of-emergency announcement on August 6, warning that extraordinary measures were needed "for the very survival of our state". Sirleaf "directed that all officials occupying ministerial level positions or equivalent -- senior and junior -- managing directors, deputy/assistant directors or equivalent, commissioners et cetera who violated the orders are hereby relieved of their positions," her office said in a statement. It did not say how many ministers were affected or who had been fired. But a government insider clarified that only deputy ministers and senior officials were involved in the dismissal, and not cabinet-level ministers. United Nations officials have pledged to step up efforts against the lethal tropical virus, which has infected more than 2,600 and killed 1,427 since the start of the year. The World Health Organization said on Monday more than 120 health workers across west Africa had died during the "unprecedented" outbreak, and more than 240 had been infected. In Sierra Leone, the WHO has temporarily pulled back its health workers from the Kailahun post after one of them was infected and an investigation is under way to prevent further infections before sending a team back there. "This was the responsible thing to do. The field team has been through a traumatic time through this incident," said Dr Daniel Kertesz, WHO representative in Sierra Leone, in a statement. And late Tuesday Nigeria announced that it was delaying the resumption of classes in all primary and secondary schools by a month due to the Ebola outbreak. "This is to ensure that adequate preventive measures are put in place before students resume," said Education Minister Ibrahim Shekarau. Meanwhile the African Development Bank warned Tuesday the epidemic could cut economic output in the three worst-hit countries, as well as neighbouring Ivory Coast, by between one and 1.5 percent of gross economic product. "If people don't start worrying about agriculture, there is going to be a food crisis. That will be the first direct impact on farmers in this region," said president Donald Kaberuka.Second Ebola front A second front has opened up in Africa's struggle with Ebola, after the Democratic Republic of Congo said on Tuesday it was preparing for a "battle of at least three months" after 13 people died after contracting the virus in the remote northeast. Congolese authorities said the outbreak concerned Zaire Ebola, the species that is ravaging west Africa, but said the two outbreaks were not linked. The United Nations' Ebola envoy David Nabarro, in Guinea's capital Conakry on Tuesday on the third leg of a tour of the region, has described the fight against the epidemic as a "war" which could take six more months. "I understand that the situation is more or less stable in Guinea," he said. "But with the surge of new outbreaks of the epidemic in recent weeks, there is an urgent need to consider a regional humanitarian rapid large-scale operation to stop the epidemic in a maximum period of six months." Guinea, where the disease was first discovered, has reported 406 deaths, Sierra Leone 392 and Nigeria five, the WHO said on Friday.Deaths likely underestimated Liberia has suffered the worst effects of the outbreak, registering 624 deaths. Sirleaf's office said she met a delegation of the United States Centers for Disease Control and Prevention, including its global head Tom Frieden, on Monday to discuss combatting the epidemic. Frieden was quoted as saying there was an urgent need for more treatment facilities. The WHO estimates its count of the infected and dead is likely far too low, due in part to community resistance to outside medical staff and a lack of access to infected areas. The Nigerian health ministry said Tuesday that two more people had been released from isolation after recovering from Ebola, leaving only one living patient with the disease in the country. Elsewhere in the region, the Ivory Coast has closed its borders with Guinea and Liberia, just days after Senegal did the same with Guinea, while South Africa has banned entry for non-citizens arriving from Sierra Leone, Guinea and Liberia. Ivorian Health Minister Raymonde Goudou Coffie told journalists in Abidjan that it was "uncanny" that the country has so far no reported cases of Ebola which is ravaging neighbouring Guinea and Liberia.