An international team of United Nations human and animal health experts has flown to Saudi Arabia to investigate a recent surge in cases of a deadly virus known as Middle East Respiratory Syndrome, or Mers.

Speaking from Riyadh on Friday, a spokeswoman for the World Health Organization-led team said it was worried by a steep rise in cases of Mers, which has infected some 50 people in the Kingdom in February alone -- one of the highest monthly rates since it first emerged in humans in 2102.

"We are all very aware of this surge in cases," said the WHO's Fadela Chaib, one of an 11-strong international Mers expert team due to end a three-day mission by Saturday.

"Although this is still a small outbreak compared to last year, we still need to understand more about what is happening."

Mers is a respiratory disease that causes coughing, fever and breathing problems, and can lead to pneumonia and kidney failure. Initial scientific studies have linked it to camels and it is known to have infected close to a thousand people, killing some 360 of them -- the vast majority in Saudi Arabia.

First identified in humans in 2012, it is caused by a coronavirus, from the same family as the one that caused a deadly outbreak of SARS (Severe Acute Respiratory Syndrome) in China in 2003. There is no cure or vaccine for Mers, which kills around 40 percent of its victims.

The WHO said earlier this month it was concerned about Mers and its potential to spread internationally.

Chaib said the international team -- including experts from the WHO, the United Nations Food and Agriculture Organization and the Paris-based World Organization for Animal Health -- were talking to scientists and doctors, going to hospitals and visiting the government's Mers command and control centre (CCC).

Saudi Arabia has been criticised by WHO and others for moving too slowly to conduct the types of scientific study needed to pin down the source of the Mers virus and to establish how it infects people and passes from one person to another.

Many people sickened by the disease catch it while in hospital, or after contact with another case, and a few also report having direct contact with camels.

"They (the Saudi authorities) are making progress, but there is a lot more work to do," Chaib said. – Reuters