A massive poultry cull is underway in China as the country announced a sixth human death from a new bird flu strain Friday.

Health authorities in Shanghai ordered the slaughter of all birds at a market where the H7N9 flu virus was found in pigeons being sold for meat. The sale of live fowl has also been halted.

Sixteen people have fallen ill with the flu along China’s eastern seaboard. Some of them are in critical condition.

So far, officials believe that the virus has been spreading only through direct contact with infected fowl. There has been no evidence to suggest that the virus has been spreading from one person to another, but scientists are monitoring the situation closely.

Allison McGeer, director of infection control at Mount Sinai Hospital in Toronto, said global health authorities will be on high alert until they can be assured that there is low risk of human-to-human transmission.

Infectious Disease Specialist Dr. Neil Rau said determining the exact source of the H7N9 virus is crucial.

“I think it is extremely important to find out the source because that is how you protect people from getting new exposures to this virus,” he told CTV News. “Even if it’s rare it’s quite deadly.”

The bird cull at the Shanghai market began Thursday night, on orders from the city’s agricultural committee. The area was cordoned off with tape and guarded by police as workers slaughtered the birds.

David Hui, an infectious diseases expert at the Chinese University of Hong Kong, told The Associated Press that the pigeons were probably affected by wild or migratory birds.

The Chinese government confirmed the latest death linked to the virus involved a 64-year-old farmer in the city of Huzhou. A 48-year-old man who transported poultry also died and several other people who have been infected were believed to have had direct contact with birds.

With files from The Associated Press