World Health Organisation (WHO) experts and local officials will visit a Saudi hospital where the SARS-like coronavirus has spread, killing seven people, the UN agency has confirmed.
France reported its first case on Wednesday in a 65-year-old Frenchman who had recently returned from Dubai with the virus that has emerged from the Gulf and has also spread to Britain and Germany as well as Jordan, Qatar and the UAE.
The French case brings total known infections worldwide to 31, of which 18 have died.
The Saudi patients appeared to have been infected in hospital with some family members also falling ill, WHO officials said.
Coronavirus is from the same viral family as triggered the outbreak of Severe Acute Respiratory Syndrome (SARS) that swept the world from Asia in late 2003, killing 775 people.
Health experts' concerns are growing over clusters of cases, despite no evidence of sustained human-to-human transmission - the type of infection pathway that can lead to pandemics.
The WHO-Saudi team of experts will focus on the Al-Moosa hospital in the town of Hofuf in Ahsa governate in Eastern Province, where the patients are being treated. Official statements released via the Saudi Press Agency have sought to reassure Saudis the outbreak is limited.
"One focal point of the investigation at the moment is the hemodialysis unit in the hospital," said WHO spokesman Gregory Hartl in Geneva, referring to equipment used on kidney and diabetes patients.
"The association of this outbreak with a single health care facility suggests nosocomial (hospital-acquired) transmission," it said, noting that some patients under treatment for ailments may have had increased susceptibility to infection.
"However, the presence of infection in two family members not associated with the facility itself raises a concern about potential broader transmission in the community," it said.
Saudi Arabia's Health Ministry spokesman did not respond to repeated phone calls seeking comment, while the director at Al-Moussa declined to comment. International media were not invited to a news conference with Health Ministry officials on Monday.
Statements on official media outlets were aimed at dispelling online speculation in Saudi Arabia that the virus had spread to other hospitals in Eastern Province.
A total of 23 cases have been reported since September in Saudi Arabia, including 13 since mid-April in al-Ahsa, where seven patients have died and four remain critically ill in intensive care, with two improving in hospital, the WHO said.
Along with the French case, the WHO has registered 23 cases in Saudi Arabia, two in Jordan, two in Qatar, one in the United Arab Emirates (UAE) and two in Britain.
France's Health Minister Marisol Touraine said the man, now on a respirator in an intensive care ward, was admitted to hospital with breathing problems and a fever on April 23, a few days after his return from a week-long trip to Dubai.
"This is an imported case and it's a unique case," Touraine told a news conference, promising a thorough investigation into how the man had become infected.
Tests were now being carried out as a precaution on all friends and relatives who had been in contact with him, but all have come up negative so far, officials said.
"He is in a critical condition. His situation is worrying," the government's health director Jean-Yves Grall told the news conference. The man, who is from the northern Nord Pas de Calais region, has needed blood transfusions, he said.
The most recent German case was in March. The man fell ill in UAE and went to Germany for treatment where he died. The previous case, in November, was a man who was infected in Qatar, treated in Germany and then discharged from hospital.