Two people in Beijing have been diagnosed with the pneumonic plague, a rare instance of the highly-contagious disease that is fatal if left untreated. The two individuals were being treated Tuesday at a hospital in China's capital city, which is home to more than 21 million people, local authorities said.

Pneumonic plague can prove fatal within 24 to 72 hours and is the "most virulent form of plague," according to the World Health Organisation (WHO), while the bubonic form is less dangerous.

The patients are from the northwestern Inner Mongolia province, district officials said in an online statement, adding that the "relevant prevention and control measures have been implemented."

The Beijing government did not respond to AFP's calls for comment, but the WHO confirmed that Chinese authorities had notified them about the plague cases.

"The (Chinese) National Health Commission are implementing efforts to contain and treat the identified cases, and increasing surveillance," said Fabio Scano, coordinator at WHO China.