His comments come as countries around the world battle to prevent the virus spreading further.
For a second day, more cases have been reported outside than inside China.
Iran and Italy have become major centres of infection, with people travelling from there spreading the virus further afield.
"It's what's happening in the rest of the world that's now our greatest concern," Dr Tedros said.
Globally, more than 80,000 people in nearly 50 countries have been infected. Nearly 2,800 have died, the majority in China's Hubei province.
What measures are countries taking?
* Japan and Iraq have ordered schools to close - a measure already taken in mainland China and Hong Kong
* Saudi Arabia is stopping foreign pilgrims from entering the country. It is not clear if the Hajj pilgrimage, which begins in July this year, will be affected
* Iran has urged citizens to avoid making unnecessary trips within the country and cancelled Friday prayers in Tehran and other cities
* Australia is extending a ban on foreign visitors from mainland China
* Italy, where 17 people have died, has quarantined 11 towns
* Greece is cancelling all carnival activities