* WHO says this outbreak is most severe, but can be stopped
* Virus has killed 961 people in current outbreak
* Nigeria declares national emergency as cases mount
* Some Liberians say anti-Ebola measures increase hardship (Adds USAID announcing new funding, patient in U.S. saying he is getting stronger; in paragraphs 14, 32-33)
By Kate Kelland and Felix Onuah
LONDON/LAGOS, Aug 8 (Reuters) - The world's worst outbreak of Ebola that has killed nearly 1,000 people in West Africa represents an international health emergency and could continue spreading for months, the World Health Organization said on Friday.
Nigeria became the third African nation, after Sierra Leone and Liberia, to declare a national emergency on Friday as the region's healthcare systems struggle to cope with the advance of one of the deadliest diseases known to man.
"The outbreak is moving faster than we can control it," WHO Director-General Margaret Chan told reporters on a telephone briefing from her Geneva headquarters.
The U.N. agency said all states where Ebola had passed from one person to another should declare a national emergency. It called the outbreak "particularly serious" but said there should be no general ban on international travel or trade.
"The declaration ... will galvanise the attention of leaders of all countries at the top level. It cannot be done by the ministries of health alone," Chan said.
In Nigeria, which has confirmed seven cases of Ebola since a man fell sick on arrival from Liberia, President Goodluck Jonathan declared a national state of emergency and approved 1.9 billion naira ($11.7 million) of emergency funds to combat the virus.
The WHO has been accused of failing to respond fast enough to the outbreak, which it said on May 18 could be declared over by May 22. It has since become more conservative in its predictions, said head of health security, Keiji Fukuda.
"At that point we thought that it was likely that it would come under control based on our experience. This outbreak has developed in ways we have not seen before," Fukuda told reporters.
"The likelihood is that things will get worse before they get better," he said, adding that the WHO is prepared for an outbreak that persists at a high level for months.
Medical charity Medecins Sans Frontieres (MSF) said the declaration showed WHO's seriousness in tackling the outbreak but added that statements should be translated into action.
"For weeks, MSF has been repeating that a massive medical, epidemiological and public health response is desperately needed to saves lives," said MSF Director of Operations Bart Janssens.
Ebola has no proven cure and there is no vaccine to prevent infection, so the best treatment is focussed on alleviating symptoms that include fever, vomiting and diarrhoea.
Its spread could be stopped with the right measures to deal with infected people, Fukuda said, but it was important that anyone known to have Ebola should be immediately isolated and treated in isolation for 30 days.
The U.S. Agency for International Development said on Friday its officials had announced more than $12 million in additional funding this week to help curb the outbreak.
The European Union said on Friday it was increasing by 8 million euros ($10.7 million) its funding to contain Ebola and governments in the affected region have taken increasingly tough measures.
The army in Sierra Leone on Thursday blockaded rural areas hit by Ebola while neighbouring Liberia declared a state of emergency. But some local people said the restrictions, while necessary, risked increasing their economic hardship.
In one example, about 30 military officers armed with AK-47s guarded a checkpoint blocking a line of trucks laden with goods from travelling from Montserrado County to the rest of Liberia.
One of the drivers, Sackie Sumo, said the closed road prevented him transporting his truckload of logs, which in turn meant he would not be paid.
"I feel bad. I need to get money to my family," he told Reuters.
Market vendor Musa Kweh whose shared taxi had been stopped at the same checkpoint, also said she was unsure how she would generate income from her goods now that the market in Monrovia she was heading to was closed.
In an attempt to make some money from potatoes and other goods, she spread them on the ground at the checkpoint to sell to passersby.
The latest WHO figures on Friday showed the worsening toll. Some 1,779 people have been infected and 961 people have died in the most severe outbreak in the 40 years since it was first identified in humans.
This was partly because of weaknesses in the countries affected, where health systems are fragile and lacking in resources. Sierra Leone has one local doctor for roughly 45,588 people while Liberia has one for every 86,275 people, according to political risk research company DaMina Advisors.
The effort to contain the disease is also undermined by inexperience and misperceptions of the threat, the WHO said.
Although most cases of Ebola are in the remote area where Guinea borders Sierra Leone and Liberia, alarm over the spread of the disease increased last month when a U.S. citizen died in Nigeria after travelling there by plane from Liberia.
The U.S. State Department has ordered the departure of family members from its embassy in the Liberian capital Monrovia and said it was acting "out of an abundance of caution".
International companies are also taking protective measures such as reducing staff in affected countries. Steel and mining company ArcelorMittal on Friday declared force majeure on a major project in Liberia because of the outbreak.
Nigeria's state oil company said on Friday it had shut down its own clinic in the Lagos commercial district of Victoria Island after a suspected Ebola case was admitted.
Airline Gambia Bird said it was now only offering one-way flights from the Gambian capital Banjul to Ebola-affected countries, following a request from the government.
Concerns have spread to other countries. Ugandan health authorities on Friday said an Ebola suspect had tested negative, while a hospital in Benin's capital was treating a Nigerian man suspected of having contracted the disease.
After an experimental drug was administered to two U.S. charity workers who were infected in Liberia, Ebola specialists have urged the WHO to offer such drugs to Africans. The U.N. agency has asked medical ethics experts to explore this option next week.
Kent Brantly, 33, a Texas doctor being treated for Ebola in Atlanta after getting evacuated from West Africa, said in a statement on Friday he was getting stronger.
"I am writing this update from my isolation room," Brantly said in his first public statement since contracting Ebola. "I am growing stronger every day, and I thank God for his mercy as I have wrestled with this terrible disease."
1 US dollar = 0.7462 euro Additional reporting by Stephanie Ulmer-Nebehay and Tom Miles in Geneva, Barbara Lewis in Brussels, Elias Biryabarema in Kampala, Alphonso Toweh in Washington and Pap Saine in Banjul; Writing by Matthew Mpoke Bigg and Emma Farge; Editing by Philippa Fletcher, Robin Pomeroy and Mohammad Zargham