BEIJING—Concerns about a deadly new strain of bird flu intensified last week as the disease claimed a sixth life in eastern China and agricultural authorities in Shanghai ordered a wide-scale slaughter of poultry in an effort to stem its spread.

The U.S. Centers for Disease Control and Prevention warned U.S. public-health departments and physicians to be on the lookout for signs of the new virus, a variant of the H7N9 strain of avian flu. The CDC said it is developing a diagnostic kit to send to U.S. states and China, and is...