A new virus from the same family as SARS which sparked a global alert in September has now killed two people in Saudi Arabia and total cases there and in Qatar have reached six, the World Health Organisation said.
The UN health agency issued an international alert in late September saying a virus previously unknown in humans had infected a Qatari man who had recently been in Saudi, where another man with the same virus had died.
On Friday it said in an outbreak update that it had registered four more cases and 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 10th of the 8,000 people it infected worldwide.
Among the symptoms in the confirmed cases are fever, coughing and breathing difficulties.
Of the six laboratory-confirmed cases reported to WHO, four cases, including the two deaths, are from Saudi Arabia and two cases are from Qatar.
Britain's Health Protection Agency, which helped to identify the new virus in September, said the newly reported case from Qatar was initially treated in October in Qatar but then transferred to Germany, and has now been discharged.
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.
It added that so far, only the two most recently confirmed cases in Saudi Arabia were epidemiologically linked - they were from the same family, living in the same household.
"Preliminary investigations indicate that these two cases presented with similar symptoms of illness. One died and the other recovered," the WHO's statement said.
Two other members of the same family also suffered similar symptoms of illness, and one died and the other is recovering. But the WHO said laboratory test results on the fatality were still pending, and the person who is recovering had tested negative for the new coronavirus.
The virus has no formal name, but scientists at the British and Dutch laboratories where it was identified refer to it as "London1_novel CoV 2012".
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.