A security alert has closed part of Newark International Airport near New York City hours after tougher screening rules for air passengers were unveiled.
Terminal C was locked down after a man entered the secure area through an exit channel without being screened on Sunday, security officials said.
The man was still being sought by police, the Associated Press reported.
Earlier, US authorities announced that more rigorous airport screening measures would begin on Monday.
The new measures are said to involve passengers from Nigeria, Pakistan, Syria, Iran, Sudan, Yemen and Cuba having pat-down body searches and their hand-baggage searched.
President Barack Obama has promised action following the alleged Christmas Day bomb plot against a US plane.
At Newark, flights were grounded and passengers evacuated from the secure side of the terminal when the unidentified man was seen entering through an exit lane, said Transportation Security Administration spokeswoman Ann Davis.
Authorities were scouring surveillance tapes to identify the man, she said.
Passenger Alison Day, 47, from York, England, said she and her fellow travellers were escorted out of the terminal's lounge but given no further instructions.
"I'm not angry that this is happening, but I'm angry that there was a lack of organisation," she said.
Terminal C mainly serves Continental Airlines flights.
US airports are on heightened alert after a failed attempt to blow up a Detroit-bound Northwest Airlines flight on Christmas Day by a Nigerian man carrying explosives in his underwear.