Saudi Arabia said that 10 more people infected with Middle East Respiratory Syndrome (Mers) had died over the last two days and identified 20 new cases of the virus, pushing the total number of infections in the country to 511.

Five of the deaths were reported on Tuesday and five on Wednesday, according to statements on the health ministry's website.

They took the death toll in Saudi Arabia to 157 since Mers, a coronavirus like Severe Acute Respiratory Syndrome (Sars), was identified two years ago.

The World Health Organisation said on Wednesday that while concern about the virus had "significantly increased", the disease was not yet a global health emergency.

Of the 16 new cases identified on Wednesday, two had died. Of the four cases identified on Tuesday, one had died, the ministry said.

Sars which killed around 800 people worldwide after emerging in China in 2002. It can cause coughing, fever and pneumonia, and there is no vaccine or anti-viral treatment against it.

The rate of infection in Saudi Arabia has surged in recent weeks after big outbreaks associated with hospitals in Jeddah and Riyadh. The total number of infections nearly doubled in April and has risen by a further 25 percent already in May.

The recent upsurge is of particular concern because of the influx of pilgrims from around the world expected in July during the Muslim fasting month of Ramadan.

Scientists around the world have been searching for the animal source, or reservoir, of Mers virus infections ever since the first human cases were confirmed in September 2012.

Meanwhile, two US hospital workers who fell ill after contact with a patient suffering from Mers have tested negative for the often-deadly virus, a Florida health official said.

US health officials had recently confirmed the country's first two cases of Mers, raising fears about the global spread of the virus.

The patient with the second confirmed case of Mers on US soil was hospitalised in Orlando, Florida after a 12-hour stay in the emergency department, potentially exposing healthcare workers to the virus. He remained in an isolation room at Dr P Phillips Hospital and had been fever free for 24 hours, the hospital said on Wednesday.

Florida officials said they were monitoring a total of 20 healthcare workers who had been in contact with the patient. Test results for 19 of those workers were negative for Mers, according to Kevin Sherin, director of the Florida Department of Health in Orange County. The 20th worker, a doctor who had left for Canada, was still awaiting test results. - Reuters