(CNN) -- Two more people in China have died from the H7N9 bird flu virus, raising the country's death toll to five, state-run news agency Xinhua reported Thursday, citing Shanghai health officials.
H7N9 is a strain of avian flu not detected in humans before China's recent cases, which the country began reporting on Sunday.
Fourteen human cases of H7N9 -- all in eastern China -- have now been reported, according to Xinhua.
"We don't know yet where the humans got their virus from," said Dr. Joseph Bresee, who heads the epidemiology and prevention branch in the Centers for Disease Control and Prevention's influenza division.
The virus has not been shown to spread easily between humans, he added.
The two most recent deaths happened Wednesday in Shanghai hospitals. All but one of the five deaths have occurred in that city.
One of Wednesday's deaths was that of a 48-year-old poultry transporter who, although he died in Shanghai, lived in nearby Jiangsu province, Xinhua reported.
The three previously reported deaths were: two men -- ages 27 and 87 -- who lived in Shanghai, and a 38-year-old man who lived in Zhejiang province but worked in Jiangsu province, according to state-run news agency Xinhua.
The CDC is working closely with Chinese authorities trying to find the source of the human infections, Bresee said.
"There are lots of things happening at CDC to prepare for this virus," Bresee said. "State health departments are readying themselves just in case," and researchers are working on developing a vaccine for this strain, he said.
CNN's Miriam Falco contributed to this report.