"People should not overreact to it or over-speculate about what's going on, but there clearly are concerns centered around aviation security that we need to be vigilant about," Johnson said late Wednesday on MSNBC.
Meanwhile, the State Department has instructed U.S. Embassy employees in Algeria to avoid U.S.-owned or operated hotels through July 4 and the Algerian Independence Day on July 5.
"As of June 2014 an unspecified terrorist group may have been considering attacks in Algiers, possibly in the vicinity of a U.S.-branded hotel," according to the message from the U.S. Embassy in Algeria.
State Department spokeswoman Jen Psaki did not provide details about the reasons for the increased security.
"I would say broadly speaking that the threat of foreign fighters is a concern that we share with many counterparts in the world, whether that's European or others in the Western world, where we've seen an increase in foreign fighters who have traveled to Syria and other countries in the region and returning," Psaki said. "And so we have been discussing a range of steps we can take in a coordinated fashion for some time."
The U.S. shared "recent and relevant" information with foreign allies, Homeland Security Secretary Jeh Johnson said in a statement Wednesday. "Aviation security includes a number of measures, both seen and unseen, informed by an evolving environment."
Southwest Airlines, which along with subsidiary AirTran Airways, flies between the U.S. and Mexico and the Caribbean, doesn't expect the directive to have much impact on its operations, spokesman Chris Mainz said. He said the focus likely would be in other parts of the world, although the airline's security personnel have been contacted by the Homeland Security Department. Mainz declined to comment on those discussions.
American Airlines spokesman Joshua Freed said the company has been in contact with Homeland Security about the new requirements but declined to comment further.
Associated Press writers Eric Tucker, Lara Jakes and Joan Lowy in Washington and David Koenig in Dallas contributed to this report.