The ancient bacterial disease, which is believed to be similar to the infection which claimed the lives of 25 million in the Middle Ages, is known to kill in less than 24 hours.
WHO’s comments come as authorities in the city of Bayan Nur, in the Chinese region of Inner Mongolia, issued a level three public health warning on Sunday, one day after a hospital reported a case of suspected bubonic plague.
Two brothers were also reported as infected with the plague in a second recent case last week, after they ate marmot meat.
Pansoch Buyainbat, 27, and his brother, 17, were being treated in separate hospitals in Khovd province in western Mongolia.
The older brother was in a “critical” condition.
The cases have now sparked urgent checks on 146 people with whom they were in contact.
There were also four reported cases of plague in the local population last November, including two of pneumonic plague, an even deadlier variant.
WHO spokeswoman Margaret Harris told a UN press briefing in Geneva: “We are monitoring the outbreaks in China, we are watching that closely and in partnership with the Chinese authorities and Mongolian authorities.
“Bubonic plague has been with us and is always with us, for centuries. We are looking at the case numbers in China. It’s being well managed.
“At the moment we are not considering it high-risk but we are watching it, monitoring it carefully,” she added.
The bubonic plague, which was known as the “Black Death’ in the Middle Ages, is a highly infectious and often fatal disease spread mostly by fleas on rodents carrying the Yersinia pestis germ.
The infection can also be spread by the human consumption of infected rodent meat such as marmots and leaves its victims with a fatal bacteria infection which creates painful enlarged lymph nodes in the neck, armpits and groin.
It can also be passed from human to human via coughs and sneezes.
It can be treated by antibiotics but if it is not treated it has a extremely high death rate of between 30-60%.
Cases are not uncommon in China although they are becoming increasingly rare.
Despite WHO’s assurances, China’s state-run People’s Daily Online has reported the city has a “level 3 warning of plague prevention and control”.
A statement read: “At present, there is a risk of a human plague epidemic spreading in this city.
“The public should improve its self-protection awareness and ability, and report abnormal health conditions promptly.”
Officials added the warning period will remain in place until the end of 2020.
The alert is the second lowest in a four-level system.