The Ebola epidemic can be stopped in its tracks, but it will take at least six to nine months and cost more than $600m (£365m), according to the director general of the World Health Organisation.

Speaking in Washington, senior WHO leaders including director general Dr Margaret Chan, called for a massive global response to the outbreak and suggested it would take longer to control than has yet been assumed.

The Ebola outbreak is "the largest, most complex and most severe we've ever seen", said Chan and spreading at a pace that outstrips effort to control it.

Guinea, Liberia, Nigeria, Senegal and Sierra Leone have between them reported about 3,500 cases and 1,900 deaths. The latest spread is to Port Harcourt in Nigeria, where it is likely that the virus will have been transmitted to many people from an infected doctor.

Médecins Sans Frontières, which says its volunteer doctors on the frontline are overwhelmed, has called on UN member states to send in teams of military and civilian biohazard experts. WHO's assistant director general for global health security, Keiji Fukuda, just back from west Africa, acknowledged that there are not enough people with the necessary skills and training to deal with all the cases in the affected region.

He said: "We don't have enough health workers, doctors, nurses, drivers and contact tracers. Most of the infections are happening in the community, and many people are unwilling to identify themselves as ill. And if they do, we don't have enough ambulances to transport them or beds to treat them yet."

Families are forced to care for patients at home, putting them and their contacts at risk of infection because of the shortage of health workers and facilities, he said. Mobilising international financial support and additional health personnel is a top priority for scaling up the international response, he added.

Airline routes to affected countries also urgently need to be re-opened. Closures "have a huge impact," Fukuda said, impeding the flow of experts and supplies into Africa, and the outbreak itself is having a negative impact on the economies of Guinea, Liberia and Sierra Leone. "People need medical supplies, food, fuel, charcoal, and other provisions. We have to get it to them."

Fukuda pointed out that WHO had not recommended any travel restrictions, "except in cases where individuals have been confirmed or are suspected of being infected with Ebola virus disease or where individuals have had contact with cases of Ebola."

Dr David Nabarro, UN coordinator for the Ebola response, said the UN system is working together on 12 detailed steps for the global response, which he estimated will cost at least $600m and require "several thousand people to scale up our response by three to four times".

Urgent needs in the affected countries include communications and messaging, care for the infected and proper burials, diagnosis and contact tracing, health services for other conditions, transport and supplies, cash to pay health workers, medical services for responders, air and sea access, and strong coordination to avert economic downturns, says WHO.