UN humanitarian chief Valerie Amos, warning that the virus could infect 20,000 people by the end of the year, said it posed a "huge funding challenge".
"If not dealt with effectively now, Ebola could become a major humanitarian crisis in countries currently affected," she told reporters in Geneva.
The capacity of worst-hit Guinea, Sierra Leone and Liberia to provide even the most basic necessities is "on the brink of collapse", she warned.
US President Barack Obama was set to outline an assistance strategy on Tuesday, including the deployment of 3,000 US military personnel and plans to train up to 500 health care providers per week in Liberia.
The United Nations said the response to the crisis would require $987.8 million (763 million euros), with about half needed for the most affected country, Liberia.
Shortly after, the World Bank approved a $105-million grant, part of a $200 million pledge made in early August aimed at helping people cope with the economic impact of the crisis and strengthening public health systems.
"The world needs to do much, much more to respond to the Ebola crisis in these three countries," World Bank President Jim Yong Kim said in a statement.
The announcements came amid mounting global alarm over the worst-ever Ebola epidemic, which by Friday had claimed 2,461 lives out of 4,985 cases, according to fresh numbers from the Geneva-based World Health Organization.
'Unprecedented' surge needed
Guinea said on Tuesday that it was postponing its Independence Day celebrations — due on October 2 -— until some point in December.
The UN document estimates that by year-end Guinea will account for 16 percent of infections, compared with 34 percent in Sierra Leone and 40 percent in Liberia.
If the international community and affected countries respond swiftly and energetically, transmission should begin to slow by the end of the year and halt by mid-2015, it said.
"The level of surge we need to do is unprecedented. It is massive," the United Nations' Ebola coordinator David Nabarro told reporters.
China will send a 59-person mobile laboratory team from its Centre for Disease Control to Sierra Leone, including epidemiologists, clinicians and nurses -- bringing the number of Chinese medics in the country to 174, the WHO said.
The EU, Britain, France and Cuba have also pledged to send medical teams and other aid to the region.
But this is far from enough, warned Joanne Liu, head of the Doctors Without Borders (MSF) charity.
Noting that the known Ebola toll "represents only a fraction of the real number," she said that "the ground response remains totally and lethally inadequate."
Liu told a meeting of UN agencies, member states and other actors involved in the Ebola fight that "the window of opportunity to contain this outbreak is closing".