"We are not standing on the sidelines while fanatics hatch new plots. The US government is focused on deterring, detecting, and disrupting these threats," Kelly said.
Passengers will be allowed to carry electronic devices larger than a cell phone onto U.S.-bound flight if they board an airline that complies with new U.S.-imposed security directives calling for more extensive passenger screening, increased use of bomb-sniffing dogs, and improved security measures.
"We could ban the devices outright," a Homeland Security official said, "But we have chosen measures that address the risk without removing the devices from airplanes."
Let our news meet your inbox. The news and stories that matters, delivered weekday mornings.
The new requirements, to be implemented in phases, will be imposed at more than 280 airports in 105 countries, affecting 180 airlines that offer direct flights to the US. An average of about 2,000 such flights come here every day, carrying a total of roughly 325,000 passengers.
Domestic flights within the U.S. will not be affected, the officials said, because security measures have been enhanced in recent months, offering a better look at the contents of carry-on baggage in general and personal electronic devices in particular.
In March, the U.S. ordered airlines operating from ten Middle Eastern and African airports to prevent passengers from carrying electronic devices on board that are larger than a cell phone, on direct flights to the United States. The directive said the devices must be stored in a passenger's checked baggage.
Homeland Security officials said the restrictions in those countries will be lifted as the affected airlines comply with the new security directives.
But if any airlines fail to comply with the new directives, the officials said, the US could levy fines, limit the carrier's ability to fly to the U.S., or completely ban electronic devices from an airline's inbound planes in both carry-on and checked baggage.
"Our enemies are constantly working to find new methods for disguising explosives, recruiting insiders, and hijacking aircraft," Kelly said. "We cannot play international whack-a-mile with each new threat. Instead we must put new measures in place across the board."
Passenger check-in procedures will vary among airports and airlines, depending on how much they need to do to meet the new requirements. Some are substantially in compliance already.
Late last year, U.S. officials say, the U.S. picked up intelligence suggesting that a terrorist group had developed the ability to conceal explosives in working laptop computers. American officials concluded that the resulting devices could successfully pass through conventional airport screening equipment.
Airlines and European airports have said extending the electronics ban would cause significant disruption and that forcing passengers to put laptop computers into their checked baggage could pose a fire hazard from the concentration of so many lithium-ion batteries in airplane cargo compartments.
A global airline industry group said earlier this month that the limited ban already in place appears to have reduced travel on the affected routes.
While worldwide air travel was up in March, it fell by about three percent from what it was a year before on routes to the U.S. flown by Middle Eastern airlines, according to the International Air Transport Association.
Alexandre de Juniac, the airline group's director general and CEO, said extending the ban to include flights from Europe would produce "a $1.4 billion hit on productivity," because passengers would not be able to use laptop computers to work in the air.
Pete Williams is an NBC News correspondent who covers the Justice Department and the Supreme Court, based in Washington.