The U.N. health agency issued a global alert in late September saying a virus previously unknown in humans had infected a 49-year-old Qatari who had recently travelled to Saudi Arabia, where another man with the same virus had died.
On Friday, it said in a disease outbreak update that it had registered four more cases and that one of the new patients had died.
"The additional cases have been identified as part of the enhanced surveillance in Saudi Arabia (3 cases, including 1 death) and Qatar (1 case)," the WHO said.
The new virus is known as a coronavirus and shares some of the symptoms of SARS, or Severe Acute Respiratory Syndrome, which emerged in China in 2002 and killed around a tenth of the 8,000 people it infected worldwide.
Among the symptoms in the confirmed cases are a fever, coughing and breathing difficulties.
Coronaviruses are typically spread like other respiratory infections, such as flu, travelling in airborne droplets when an infected person coughs or sneezes.
The WHO said investigations were being conducted into the likely source of the infection, the method of exposure, and the possibility of human-to-human transmission of the virus.
"Close contacts of the recently confirmed cases are being identified and followed-up," it said.
The WHO urged all its member states to continue surveillance for severe acute respiratory infections.
"Until more information is available, it is prudent to consider that the virus is likely more widely distributed than just the two countries which have identified cases," it said.