The World Health Organisation has declared the Ebola outbreak an international public health emergency, but it is not recommending general bans on travel or trade.

The global body said the Ebola outbreak – the largest and longest in history – was happening in countries without the resources to manage the infections, some with devastated healthcare systems still recovering from war, and called on the international community to help.

"Countries affected to date simply do not have the capacity to manage an outbreak of this size and complexity on their own," said Margaret Chan, the WHO's director general. "I urge the international community to provide this support on the most urgent basis possible."

The current outbreak began in Guinea in March and has spread to Sierra Leone and Liberia, with some cases in Nigeria. There is no licensed treatment or vaccine for Ebola and the death rate has been about 50%.

The virus has an incubation period of up to 21 days, meaning symptoms do not necessarily show before then.

The WHO emergency committee unanimously agreed, after two days of meetings in Geneva and teleconferences with representatives in Guinea, Liberia, Sierra Leone and Nigeria, that the outbreak was "an extraordinary event", meeting all the conditions for a public health emergency, Chan said.

With 1,711 confirmed and suspected cases, and 932 deaths, the WHO said the outbreak was a public health risk to other states – particularly in view of "fragile health care systems" in the affected countries.

Although the WHO said that "there should be no general ban on international travel or trade," it issued a long list of recommendations on travel and contacts, including urging that all travellers leaving the countries affected by the outbreak should be screened for fever, and that no corpses should be transported across borders.

It said other states should provide information to people travelling to affected and at risk areas, be prepared to detect, investigate and manage Ebola cases, and be prepared for the evacuation and repatriation of nationals, including health workers.

States should also ensure access to specialist diagnostic laboratories, and prepare to manage travellers who arrive at international airports or border crossings with "unexplained febrile illness".

"The possible consequences of further international spread are particularly serious in view of the virulence of the virus, the intensive community and health facility transmission patterns, and the weak health systems in the currently affected and most at-risk countries," a statement said. "A coordinated international response is deemed essential to stop and reverse the international spread of Ebola."

The charity Save the Children, which said it was scaling up its operations in the region, warned that medical services in the affected countries were already overwhelmed. Rob MacGillivray, its regional humanitarian director, said that even before the outbreak there was less than one doctor for every 33,000 people in Sierra Leone and Liberia.

"Parents are understandably frightened and stay away from medical centres through fear of coming into contact with the infection. Pregnant mothers are giving birth at home rather than seeking skilled help and orphaned children are at risk of being ostracised from their communities at the most vulnerable time in their lives.

"Challenges remain in reaching families in rural communities who were struggling to access healthcare even before the outbreak."

The WHO said health advice at airports and ports or border crossings should warn travellers that though the disease is rare, careful hygiene should be practised, and all contact with blood and body fluids of infected people or animals, or with any items that have come in contact with such blood or body fluids, must be avoided.

It also says that sexual intercourse with a sick person or one recovering from Ebola should be avoided "for at least seven weeks".

The WHO advises that the risk to travellers from sharing a flight with somebody who is showing symptoms of Ebola is "very low" – but does recommend contacting fellow travellers if a sufferer reports their condition and seeks medical help on arrival.

For those travelling to affected areas, the WHO describes the risk of business travellers or tourists returning with the virus as "extremely low" – even, it says, "if the visit included travel to the local areas from which primary cases have been reported".

"Transmission requires direct contact with blood, secretions, organs or other body fluids of infected living or dead persons or animals, all unlikely exposures for the average traveller. Tourists are in any event advised to avoid all such contacts."

It said the risk to travellers visiting friends and relatives in affected countries was similarly low "unless the traveller has direct physical contact with a sick or dead person or animal infected with Ebola virus".

The long list of advice to affected states includes screening all travellers leaving for fever, banning the remains of those who have died of Ebola from being transported across borders, and ensuring "funerals and burials are conducted by well-trained personnel".

Countries with land borders with the affected states are urged "urgently to establish surveillance for clusters of unexplained fever or deaths due to febrile illness", and to act within 24 hours of any suspected cases.

The United States is sending teams of experts to Liberia, including 12 specialists from the US Centers for Disease Control and Prevention, after the Liberian president, Ellen Johnson-Sirleaf, declared a 90-day state of emergency and said the disease had overwhelmed her country's healthcare system.