DUBAI (REUTERS) - A nurse in a hospital that held France's only confirmed case of the Sars-like coronavirus that has killed 18 people has been admitted to hospital in northern France on suspicion on being infected herself, French health officials said on Friday.

The World Health Organisation (WHO) raised the number of cases confirmed worldwide to 33 after Saudi Arabia said that two people who were admitted to hospital there in April had been determined by laboratory analysis to be infected.

There is no evidence so far of sustained human-to-human transmission of the coronavirus. But health experts are concerned about clusters of new possible cases of nCoV, which started in the Gulf and spread to France, Britain and Germany.

Two people who had contact with the confirmed French case, a 65-year-old man who fell ill on returning from Dubai, were admitted to hospital late on Thursday.

One was a patient who shared a ward with him when he was in a hospital in the town of Valenciennes, northern France, at the end of April, and the other is a doctor who treated him there.

The 65-year-old, who is in stable but serious condition, was transferred to an isolated intensive care wing in Douai near Lille, which is where the third case appeared. He was transferred to Lille on Thursday night.

"We identified it overnight. It corresponds to the investigations we've been undertaking since our confirmed case of the coronavirus," said Dr Sandrine Kueny, deputy director of the regional health agency.

The nurse worked in the hospital's infectious disease unit but it was unclear whether she had direct contact with the sick man.

The coronavirus is from the same viral family that triggered the outbreak of Severe Acute Respiratory Syndrome (Sars) that swept the world after starting in Asia in late 2003 and killed 775 people.

Giving details of the two new Saudi cases, Saudi deputy health minister Ziad Memish said in an e-mail message to the web-based disease monitoring system ProMED that a study of earlier reported cases and repeat testing of suspected cases had identified two additional cases on May 8.

The first patient was a 48-year-old man with multiple coexisting medical conditions who became ill on April 29. He was in stable condition, the WHO said.

The second patient was a 58-year-old man with an existing medical condition who became ill on April 6. He fully recovered and was discharged from hospital on May 3.

Dr Memish added in the email that actions taken by Saudi authorities since May 1 had prevented new cases emerging.

Since the beginning of May, 15 patients have been reported from the Saudi outbreak, of which seven had died, the WHO said.

Of the 15 patients, 12 were men and three women. The age range of the patients are from 24 to 94 years old.

French authorities advise that anyone who has recently travelled to the Gulf region consult a doctor in case of fever.