"This is the first time health care workers have been diagnosed with nCoV (novel coronavirus) infection after exposure to patients," the WHO said in a statement.
The two health care workers were among six new cases announced by the Saudi health ministry on Tuesday.
The UN's health body said that while other health care workers had contracted the deadly disease in Jordan, there had until now not been clear evidence that they had been infected by patients carrying the virus.
"This is the first time we have pretty hard and fast evidence of it," WHO spokesman Gregory Haertl explained to AFP.
"Health care facilities that provide care for patients with suspected nCoV infection should take appropriate measures to decrease the risk of transmission of the virus to other patients and health care workers," the agency said.
WHO said one of the new patients with laboratory-confirmed nCoV was a 45-year-old man who became ill on May 2, and who was currently in a critical condition.
The second patient was a 43-year-old woman with a coexisting health condition, who became ill on May 8 and was in a stable condition.
"Health care providers are advised to be vigilant among recent travellers returning from areas affected by the virus who develop SARI," or severe acute respiratory infections, WHO said.
The organisation however continued to say travel restrictions and special screening was not yet called for to limit the spread of the virus.
Since last September, WHO says it has been informed of a global total of 40 laboratory confirmed cases of the virus, including 20 deaths.
While the virus has been deadliest in Saudi Arabia, which now counts 30 infections, half of them fatal, cases have also been reported in Jordan, Qatar, Germany, Britain and France, where two patients are now in hospital in the northern city of Lille.