The UN health agency on Thursday declared an international emergency over the deadly coronavirus from China -- a rarely used designation that could lead to improved international coordination in tackling the disease.

"Our greatest concern is the potential for the virus to spread to countries with weaker health systems.... This is not a vote of no confidence in China," World Health Organization (WHO) chief WHO chief Tedros Adhanom Ghebreyesus told a briefing in Geneva.

The alert is no reason for over-reacting, Tedros added.

"We don't recommend trade and travel restrictions as the WHO," Tedros told a press conference, arguing that such steps would unnecessarily interfere with commerce and transport.

Despite China's massive and successful efforts to limit the spread of the virus abroad, an international health emergency was declared because of the risk that the virus poses to countries with weak health systems.

"We are all in this together and we can only stop it together," Tedros said, advocating international cooperation to aid such developing countries; work on vaccines and diagnostics; and review public health plans.