UNITED NATIONS, Sept. 2  -- With the "largest and most severe" Ebola virus outbreak advancing in West Africa, the battle against it is losing ground but "this outbreak can and will be controlled," UN officials said Tuesday.

UN Deputy Secretary-General Jan Eliasson, Dr. Margaret Chan, chief of the UN's World Health Organization (WHO), and Dr. David Nabarro, the UN system's senior coordinator in the battle against the disease, were among UN officials briefing the UN General Assembly. They later briefed reporters here.

"This is one of the most serious health challenges we have ever seen in the United Nations," Eliasson said, calling the crisis " critically serious and deeply worrying."

"Not only is it a health crisis but increasingly, a developmental, humanitarian and security crisis, potentially," he said. "The whole UN system is mobilized for action and not only the UN system as such but we will growingly be dependent on actions taken by member states."

"This will be a test of capacity and a test of multilateralism and solidarity with people in need," the deputy secretary-general said, warning that time was critical.

"This is the largest and the most severe and most complex Ebola (outbreak) ever seen in the nearly 40 year history of the disease, " said Chan. "No one, even outbreak responders, even dating back to 1976 to 1995, who have managed those outbreaks, have never seen anything like it."

"Now the outbreaks .. are racing ahead of the control efforts," she said.

"Ebola has become a global threat which requires urgent global efforts in solidarity with the affected countries," the WHO chief said. "The national authorities should take the lead by coordinating all the actors that they can bring to bear that can help us to control the outbreak. That speaks volumes about the need to a well coordinated mechanism, platform to bring together partners."

"This outbreak can and will be controlled," she said. "We know what is needed and we know how to do it."

Nabarro, who is charged with coordinating efforts not only within the UN system but also with other international organizations, such as the International Committee for the Red Cross (ICRC) and Doctors Without Borders (MSF), also was quick to draw a dark prognosis for the immediate future but still expressed optimism, like Chan, for controlling the epidemic.

"In the UN system our job is to catalyze and support ways in which the generosity of all nations can be brought to the communities that need it the most," he said. "For that we will establish platforms at many levels in order to bring in unprecedented support in a dignified and supportive way."

"The outbreak is advancing ahead of us," Nabarro said. "It is accelerating ahead of us."

"We, in our control efforts, collectively, are falling behind," he said. "The more we fall behind and the more the outbreak accelerates ahead of us the harder it is to overtake it and control it."

The experts at the briefing discussed the need for "a major surge in the response."

"Why we want to see that surge in place is so that we can get back in line with the outbreak and ahead of it as quickly as possible," Nabarro said.

He listed 12 points to "this exceptional response," the first of which is to improve understanding of the issue and to mobilize societies around that better understanding of Ebola disease.

"We require responsibility from the whole world with what is a global emergency," said Nabarro.