WASHINGTON (AP) - Building on requirements imposed after the 2001 terrorist attacks, the Federal Aviation Administration on Friday proposed new rules to make it even harder for potential hijackers to force their way into the cockpits of commercial jets.

The plan would require jet makers to design aircraft with stronger floors, bulkheads and ceilings to better protect the crew, as well as install a reinforced bomb closet where a bomb could be placed if one is found during flight.

The proposed rules would apply only to planes built after the rules take effect - expected later this year or next - and would not require any retrofitting of existing planes, said FAA spokeswoman Alison Duquette.

Since 2003, federal law has required that every large commercial plane flying in the United States has bulletproof cockpit doors. Federal regulations also require that cockpit doors be locked during flight.

"Following what we did after 9/11 replacing cockpit doors, this takes it a step further by improving areas such as bulkheads and floors in front of the cockpit," Duquette said.

Officials from the world's two largest airplane makers - Chicago-based Boeing Co. and European jet maker Airbus SAS - said they would study the proposed rules.

"We're committed to working with the FAA and our customers to enhance the safety and efficiency of our air transport fleet," said Boeing spokesman Jim Proulx. "That said, we look forward to reviewing the proposed rules and offering our comments to the FAA."

Mary Anne Greczyn, a spokeswoman for Airbus North America, said Airbus will continue to work with the FAA to improve airplane safety.

"Securing the safety of the flying public is a communal effort involving many agencies and organizations around the globe. Airbus will continue to work as part of the commercial aviation community to determine if any further actions are appropriate," she said.

The FAA, in its proposal, also called for improved smoke and fire suppression systems and other changes to make it harder to hide explosives or weapons in the cabin. The agency also wants airliners to have a reinforced compartment where a bomb could be placed if one is found during flight.

"This proposal would decrease aircraft vulnerability and increase aircraft survivability in the event of a bombing or hijacking," the agency said. The plan could cost manufacturers and airlines more than $450 million (euro344 million) over dozens of years.