Law-enforcement officials and retailers are investigating a recent wave of bomb threats across the United States, targeting grocery operators and other stores, the Wall Street Journal reported on Sunday.

Retail companies including Kroger, Walmart and Amazon's Whole Foods Market, among others, have received bomb threats at their stores in recent months, the report said, adding that some callers demanded gift cards, bitcoin or money and threatened to detonate bombs if payments were not made.

These threats have been spread across various areas from New Mexico to Wisconsin. At a Kroger-owned store in New Mexico, an employee received a call from a suspect who asked her to wire money and said a bomb would go off if she called the police.

A similar incident was reported in a suburb north of Chicago, where a caller told a Whole Foods Market employee a pipe bomb had been placed in the store and demanded $5,000 in bitcoin, according to the Wall Street Journal.

The FBI said it is working with local and state law-enforcement officials and asking members of the public to maintain awareness of their surroundings and report any suspicious activity to law enforcement, the newspaper added.

According to the report it is unclear to authorities whether the threats are part of an organized effort.

(Reporting by Shivani Tanna in Bengaluru, editing by Ros Russell)