DAKAR, Senegal -- An independent panel of experts said Monday that "deep and substantial" change is needed at the World Health Organization following its slow response to early warnings about West Africa's Ebola outbreak.

After studying what went wrong in the WHO's response to the Ebola crisis, the panel said it's still not clear why the global body took so long to heed warnings.

More than 11,000 people have died from the disease since the first cases became public in rural Guinea in March 2014. In a memo that month, the WHO itself noted that the outbreak was very unusual.

"It is still unclear to the panel why early warnings, approximately from May through to July 2014, did not result in an effective and adequate response," the experts wrote in their first report issued Monday.

"Although WHO drew attention to the 'unprecedented outbreak' at a press conference in April 2014, this was not followed by international mobilization and a consistent communication strategy."

Internal documents obtained by The Associated Press earlier this year detail how WHO resisted declaring the Ebola outbreak a public health emergency until months after staff considered raising the alarm.

The documents show WHO's top leaders were informed of how dire the situation was but resisted declaring an emergency in part because it could have angered the countries involved, interfered with their mining interests or restricted the Muslim pilgrimage to Mecca.

The emergency was eventually declared on August 8, when nearly 1,000 people had already died from Ebola.

WHO has acknowledged acting too slowly. In its defence, the agency says the virus' spread was unprecedented and blames factors including lack of resources and intelligence from the field. WHO's Ebola response leader, Dr. Bruce Aylward, couldn't say whether an earlier declaration would have made a difference.

The panel of experts plans to travel to the affected countries in the coming weeks.

Ebola continues to spread in the West African nations of Guinea and Sierra Leone, while Liberia was declared Ebola-free on Saturday.