BEIJING/GENEVA (UPDATED) - The World Health Organization (WHO) said on Thursday it was declaring the China coronavirus outbreak that has killed 170 people in China a global emergency, as cases spread to 18 countries.

The United States reported its first case of person-to-person transmission, the fifth country outside China to do so. Experts say cases of person-to-person transmission outside China are especially concerning because they suggest greater potential for the virus to spread further.

Tedros Adhanom Ghebreyesus, WHO director-general, told a news conference in Geneva that recent weeks have witnessed an unprecedented outbreak which has been met by an unprecedented response.

"Let me be clear, this declaration is not a vote of no confidence in China," he said. "Our greatest concern is the potential for the virus to spread to countries with weaker health systems."

The declaration of a global emergency triggers recommendations to all countries. It is aimed at preventing or reducing cross-border spread of disease, while avoiding unnecessary interference with trade and travel.

The vast majority of the more than 7,800 cases detected globally, according to the latest WHO data, have been in China, where the virus originated in an illegal wildlife market in the city of Wuhan.

But nearly 100 cases have emerged in other countries, spurring cuts to travel, outbreaks of anti-China sentiment in some places and a surge in demand for protective face masks. - with a report from Agence France-Presse