The new Sars-like respiratory illness which has killed more than half of the people who have been infected with it is a "threat to the entire world", the World Health Organisation (WHO) has warned.
Experts raised concerns that the disease is "emerging faster than our understanding".
The WHO says that since September last year there have been 49 laboratory confirmed cases across eight countries which have resulted in 27 deaths, including two people in the UK.
Officials from the Ministry of Health in Saudi Arabia today confirmed a further five cases of the novel virus.
Earlier this week, WHO's director general said that Middle East respiratory syndrome coronavirus, or MERS-CoV, is her "greatest concern".
Addressing the World Health Assembly in Geneva on Monday, Dr Margaret Chan said: "Looking at the overall global situation, my greatest concern right now is the novel coronavirus.
"We understand too little about this virus when viewed against the magnitude of its potential threat.
"Any new disease that is emerging faster than our understanding is never under control.
"These are alarm bells and we must respond. The novel coronavirus is not a problem that any single affected country can keep to itself or manage all by itself.
"The novel coronavirus is a threat to the entire world."
Most of the patients who became infected had travelled to the Middle East but earlier this month health officials confirmed that the virus can transmit from person to person.
Health workers in the UK have been advised to be vigilant for severe unexplained respiratory illness in anyone who has
recently travelled in the region, as well as any unexplained clusters of such illness.
Infected patients have presented with serious respiratory illness with fever, cough, shortness of breath and breathing difficulties.
Coronaviruses cause most common colds but can also cause Sars (severe acute respiratory syndrome). In 2003, hundreds of people died after a Sars outbreak in Asia.