Cows at three dairy farms in California, the top US milk-producing state, tested positive for bird flu, Reuters reported, citing the state's agriculture department on Friday.

The infections expand a US outbreak of the H5N1 virus in dairy cattle to a 14th state. More than 190 herds have been infected nationally since March, along with 13 dairy and poultry farm workers, according to federal data.

The virus's jump to cows from birds has heightened concerns it could adapt to spread among humans. Federal officials say bird flu is a low risk for the general public and pasteurization inactivates the virus in milk.

No human cases have been confirmed in California, the California Department of Food and Agriculture said in a statement. The infected dairy herds, located in the state's Central Valley region, began showing symptoms on Aug. 25, the statement said.

Infected cows often suffer temporarily from reduced milk production.

"This is a tough time for our dairy farmers given the economic challenges they're facing," said Karen Ross, California's agriculture secretary.

"We are approaching this incident with the utmost urgency.”

The infected dairies have been quarantined, the state agriculture department said. Sick cows are isolated and being treated at the farms, while healthy cows have been cleared to continue shipping milk for pasteurization, the department said.

The US Department of Agriculture said earlier it was conducting tests to confirm the cases in California. USDA said it is in close contact with the California Department of Food and Agriculture as part of efforts to contain the virus.