Sharesof Humana Inc. dropped sharplyin afternoon trading July 7, amid fresh concerns that its planned with Aetna Inc. could face federal antitrust objections.

The healthinsurer's stock fell 10.70% to $160.73 as of 3:31 p.m. ET. Aetna's shares also slidthroughout the afternoon, down 4.53% to $114.79.

Antitrust-focusedpublication MLex reported the same day that the U.S. Justice Department is skepticalof the potential tie-up, which would join two of the nation's largest publicly tradedhealth insurers. Federal officials are concerned that the deal could hurt competitionwithin the industry, with Bloomberg News reportingthat they will discuss the possibility of Aetna selling some of its assets duringa July 8 meeting.

The meetingcomes soon after Justice Department officials expressedserious concerns about Anthem Inc.'smultibillion-dollar deal to acquire CignaCorp.

Humana'splanned merger with Aetna is undergoing a federal antitrust review and individualreviews in states where the deal could significantly affect their respective healthinsurance markets. The Missouri Insurance Department in May became the first stateagency to object to thedeal, finding that it would violate competitive standards.