The Health Service Executive is advising against all non-essential travel to Liberia, Sierra Leone, Guinea and Nigeria because of the threat posed by Ebola.

The advice was issued in response to the World Health Organization's declaration earlier today that the outbreak is an international public health emergency.

The WHO also said the outbreak, affecting four countries in Western Africa, is an extraordinary event posing a public health risk to other states.

The HSE said travellers returning from affected areas are advised to seek medical attention should they develop sudden fever, unexplained fatigue, diarrhoea or have other symptoms within 21 days of departure from any of the affected areas.

The WHO has said a coordinated response was needed to combat the virus, which has killed more than 900 people in Liberia, Sierra Leone, Guinea and Nigeria.

The comments came after the WHO held a two-day emergency meeting in Geneva.

It said the possible consequences of the further international spread of Ebola are "particularly serious" due to the virulence of the virus.

However, it said there should be no general ban on international travel or trade due to the outbreak.

The United States last night ordered families of its diplomats in Liberia to leave and warned against non-essential travel to the country because of the growing Ebola outbreak.

A State Department statement said US staff would remain on active duty at the embassy.

It said additional staff were being sent to help the government tackle the outbreak of the deadly virus.

"The latest wave of the outbreak has overwhelmed Liberia's health system and most health facilities lack sufficient staff or resources to address the continuing transmission of the disease," it said.

Extra US personnel going to Monrovia include 12 disease prevention specialists from the Centers for Disease Control and Prevention.

A 13-member disaster assistance response team from USAID to help the government fight the outbreak is also being deployed, State Department spokeswoman Marie Harf said.

Nigeria's President Goodluck Jonathan today declared a national state of emergency.

Liberian President Ellen Johnson-Sirleaf announced a state of emergency on Wednesday effective for 90 days that allows the government to curtail civil rights and deploy troops and police to impose quarantines on badly-affected communities.

However, it said there should be no general ban on international travel or trade due to the outbreak.

Greece has said it is running tests on a man suspected of carrying the Ebola virus.

The Greek man, an architect who had recently travelled to Nigeria, was undergoing tests at an Athens hospital, a health ministry spokesman said.

"It is likely that the man checked himself in," he said, declining to give further details on the patient or the case.

The ministry had earlier identified the patient as Nigerian.

Another feared Ebola case involving another Greek who had travelled to Nigeria was investigated earlier this week and turned out to be malaria, he added.

Greece has warned airport and port staff to be vigilant, but the state health organisation Keelpno has insisted that chances of an outbreak in the country are low.

Meanwhile, Uganda has said it had placed a passenger with Ebola-like symptoms in isolation.

"There is a suspected case, samples have been taken from the suspect and we are analysing them," said Ugandan health ministry spokesman Rukia Nakamatte.

The passenger is the first to be tested in east Africa, although Uganda has suffered Ebola outbreaks in the past, most recently in 2012.

The patient was stopped as he arrived in Uganda's main Entebbe Airport showing signs of fever. It is reported that he was travelling from South Sudan.