Airport security has already been stepped up
The US is taking additional air security measures in the wake of last month's airliner bomb plot, a senior official has said.
Homeland Security Secretary Janet Napolitano said the measures included enhanced random screening and more air marshals on some routes.
Ms Napolitano referred to the "continued threat" from al-Qaeda.
The US had already boosted security following the attempted attack on a trans-Atlantic jet on 25 December.
A 23-year-old Nigerian, Umar Farouk Abdulmutallab, has been accused with trying to detonate a bomb on a flight to Detroit and has been charged with the attempted murder of 290 people. He has pleaded not guilty.
Air marshals and better screening were among the measures announced by US President Barack Obama after an intelligence review last week.
Ms Napolitano said on Thursday that the US was "taking an additional set of aviation security precautions to protect the American people".
"Some of these measures include enhanced random screening, additional federal air marshals on certain routes and adding individuals of concern to our terrorist watch list system," she said.
"As a result of these measures and others we have put in place since Christmas, travellers should allot extra time when flying - particularly into the United States from overseas."
Mr Obama has criticised "systemic" intelligence failings over the plot.
Last week he announced that he had ordered an immediate strengthening of the terrorist watch list, information on security risks would be distributed more widely, and analysis of that information would be improved.
Mr Abdulmutallab's name was on a US database of about 550,000 suspected terrorists, but not on a list that would have subjected him to extra screening or prevented him from boarding a flight to the US.
He caught the flight to Detroit from the Netherlands, after connecting from a flight from Nigeria.
Al-Qaeda in the Arabian Peninsula, which is based in Yemen, claimed responsibility for the failed bomb attack.