World leaders gathered at the United Nations heard dire warnings and desperate pleas for assistance Thursday as the deadly Ebola virus forced Sierra Leone to quarantine a million people.

US President Barack Obama led calls for a ramped up response to the growing West African outbreak, urging governments, businesses and international organizations to join the fight.

The United Nations did win fresh pledges of support, and the Group of Seven nations announced it would keep open vital air and sea links with Ebola-hit countries in West Africa.

But Obama warned: "We are not doing enough" -- and UN officials said a 20-fold surge in assistance is needed to come to grips with an outbreak that has killed close to 3,000 people

"Right now, everybody has the best of intentions, but people are not putting the kinds of resources necessary to put a stop to this epidemic," he said, on the sidelines of the UN General Assembly.

- 'Ebola is winning' -

Health experts agreed. "Today, Ebola is winning," said Joanne Liu, president of the aid group Doctors Without Borders (MSF).

"The reality on the ground today is this: the promised surge has not yet delivered," she warned.

Health systems in the worst-hit countries of Liberia, Sierra Leone and Guinea have been overwhelmed by the epidemic and are in dire need of doctors, nurses, medical equipment and supplies.

Canada announced a contribution of $27 million dollars to the effort and France said it has set aside 70 million euros in a battle that the United Nations estimates will require close to one billion dollars.

The European Union said it would add 30 million euros to the current 150 million euros it has provided to fight Ebola.

UN officials could not provide an immediate tally of the total pledges made at the UN meeting but the UN's coordinator for Ebola, David Nabarro, said countries had "responded with generosity."

- 'Disease of the world' -

Sierra Leone on Thursday took the drastic step of putting more than one million people in five districts under quarantine-- the largest open-ended lockdown in the Ebola outbreak.

"My country is at the battlefront of one of the biggest life and death challenges facing the global human community," Sierra Leone's President Ernest Bai Koroma warned by video link from Freetown.

"Ebola is not only a disease of Sierra Leone and its neighbors, it is a disease of the world."

The northern districts of Port Loko and Bombali have been closed off indefinitely along with the southern district of Moyamba -- effectively sealing in around 1.2 million people.

With the eastern districts of Kenema and Kailahun already under quarantine, more than a third of the population of six million -- in five of the nation's 14 districts -- now finds itself unable to move freely.

In New York, the G7 foreign ministers pledged to work to maintain transport links with the Ebola-hit countries to allow much-needed help to reach those in need.

"We warn that although the spread of Ebola must be contained, affected countries must not be isolated," said the G7 -- Britain, Canada, France, Germany, Italy, Japan and the United States.

Several airlines moved in late August to suspend flights to the three affected countries, although some have since resumed.

A UN mission on Ebola set up last week is due to deploy in West Africa on Sunday, bringing supplies and equipment including protective suits, trucks, helicopters and other aircraft.

The United States is sending 3,000 troops to Liberia to help battle the contagion and has mobilized its experts from the Centers for Disease Control and Prevention to help beat back the virus.

The CDC estimates that cases in Liberia and Sierra Leone could rocket to 1.4 million by January -- in a worst-case scenario based on data obtained before the world ramped up its response.

World Bank president Jim Kim said the "spread, the magnitude and the complexity of the Ebola crisis is like nothing we have ever seen" and warned it could lead to the "potential meltdown of the continent."