Ebola is spreading at the rate of five new cases an hour in Sierra Leone, according to figures released as world leaders and experts on disease control gathered in London to discuss the outbreak.

The figures from Save the Children showed there were 765 new cases last week in the west African country alone, but only 327 hospital beds to treat infected patients. The charity said the “terrifying” rate of the spread of the disease was outstripping medical supplies and threatened a breakdown of Sierra Leone’s already fragile health system.

The rate of spread of the deadly virus is projected to double to 10 people an hour in the country before the end of October, Save the Children said.

The UN said the spread of the disease in neighbouring Liberia was just as alarming and called for a massive international response to prevent the outbreak wreaking havoc in west Africa and beyond. Speaking from Liberia, Anthony Banbury, the head of a new UN Mission for Ebola Emergency Response, said: “It is fairly similar in Liberia. The disease is spreading very rapidly – cases doubling every 20 days.”

Interviewed on BBC Radio 4’s Today programme, Banbury said: “The disease has reached every county of Liberia. It is clear that the international community has to have a rapid and very strong response to get this disease under control before it wreaks much more massive havoc in these countries and possibly other ones.”

The first patient to be diagnosed with Ebola outside of Africa told medical staff in the US that he had recently travelled to Liberia.

Banbury added: “We now need a very massive international response, that combines health interventions, big logistics as well as mass community mobilisation – getting information out to very remote villages, explaining to them how to protect themselves against the disease and what to do if someone falls sick.”

Save the Children’s chief executive, Justin Forsyth, said: “The scale of the Ebola epidemic is devastating and growing every day, with five people infected every hour in Sierra Leone last week. We need a coordinated international response that ensures treatment centres are built and staffed immediately.

“This is not only an immediate humanitarian threat, but risks completely undoing the hard work which has been done to build up fragile health systems in Sierra Leone and Liberia after the devastating wars of the past few decades. ”

Britain hosted a conference to discuss the international response to the outbreak on Thursday morning.

The British foreign secretary, Philip Hammond, and development secretary, Justine Greening, were joined by representatives of five African countries and ten other nations including the USA and Cuba, which is sending 300 doctors and nurses, at Lancaster House. UN organisations, charities, the British actor Idris Elba and William Pooley, the British nurse who recently recovered from Ebola, were also present.

Hammond said during a break that Ebola was “a terrible scourge”. The conference “was designed to galvanise the international community into greater action to tackle the disease in Sierra Leone... The UK is leading and co-ordinating responses but we need international help. Today we have made significant progress towards securing it.” Sustainable action was needed for at least the next three or four months, he said.

Ebola spread is alarming, says foreign secretary Philip Hammond Guardian

He paid tribute to the courage and sacrifice of Pooley, who told the conference of his experiences treating those with the disease in Sierra Leone. Pooley was in the US earlier in the week giving his blood, which now contains antibodies against Ebola, to help vaccine researchers in their work, said Hammond, and is now helping to train health workers heading to west Africa. He is “acting as an ambassador for the crusade against Ebola”.

Greening rejected criticism from MPs on the international development committee who said in a report that cuts in bilateral aid to the region had undermined health services and contributed to the spread of Ebola. The UK gave £51m in 2010 and £68m in 2012, she said, as well as contributing to the Global Fund and GAVI, which channel money into drugs and vaccines. The committee said the crisis “demonstrates the dangers of ignoring the least developed countries in the world”.

The UK has now invested £125m in the fight against Ebola, including the promise of 700 additional beds, in hospital units being constructed under the supervision of the military, and 400 NHS staff have so far volunteered to go out to help staff them and train local people.