The U.S. Department of Transportation said Tuesday it may seek additional conditions to protect domestic competition after giving Continental Airlines Inc. (CAL) tentative approval to join the global Star Alliance.

Continental received clearance from the DOT to join Star - led by United Airlines, a unit of UAL Corp. (UAUA), and Deutsche Lufthansa AG (LHA.XE) - later this year, after opting to defect from the rival SkyTeam grouping.

Delta Air Lines Inc. (DAL), a leading member of SkyTeam, had called on DOT to restrict the antitrust approval granted to Continental and its new partners.

While the enlarged Star grouping focuses on international routes, Delta had raised concerns about the impact of Continental's entry on domestic routes.

Delta had even called, in a regulatory filing in December, for Lufthansa to drop its board seats on JetBlue Airways Inc. (JBLU) and "restructure" its 19% stake in the carrier to secure regulatory approval for enlarging Star.

JetBlue is not a member of Star.

DOT referenced Delta's concerns in its show-cause order Tuesday giving approval for Continental's switch, saying: "We will therefore consider whether additional safeguards are necessary to preserve domestic competition."

-By Doug Cameron, Dow Jones Newswires; 312-750-4135; doug.cameron@dowjones.com