The American Civil Liberties Union says more stringent screening for airline passengers from 14 nations are ineffective and unconstitutional.
It says they constitute racial profiling when there is no realistic way to predict the national origin of a potential attacker.
The tighter security measures, announced by the United States Transportation Security Administration, are part of a crackdown after the failed bombing of a Detroit-bound flight.
The 14 nations include four listed as state sponsors of terrorism - Iran, Sudan, Syria and Cuba.
The others are Afghanistan, Algeria, Iraq, Lebanon, Libya, Nigeria, Pakistan, Saudi Arabia, Somalia and Yemen.
Security stepped up
From 4 January, there will be enhanced screening of certain passengers flying to the US from or via the 14 countries.
There will be more stringent checks of passengers' carry-on bags and full body pat-down searches. In addition, people travelling to the US from any other country could be subjected to further checks at random.
Nigeria has criticised the new security measures as unfair and discriminatory. Information Minister Dora Akunyili says it is wrong to discriminate against more than 150 million people because of the behaviour of one person, AFP reports.
Meanwhile, British airport operator BAA says it will move quickly to install full-body scanners at London's Heathrow Airport.
The scanners will be introduced within weeks at Heathrow, Europe's busiest airport, and will be used for flights to all destinations.
Scanners will also be introduced at other airports throughout Britain.
New Zealand's Aviation Security Service says extra security controls that have been in place for passengers traveling to the US since 26 December will remain.
Aviation Security Service general manager Mark Everitt told Radio New Zealand's Summer Report there are also additional checks announced, including those regarding passenger history, which are being worked out.
Meanwhile, a growing number of foreign embassies in Yemen have closed or restricted public access as a precaution against possible attacks by al Qaeda-linked militants.
France has joined the US and Britain in shutting its embassy in the capital Sana'a, while the German and Japanese missions are conducting only essential business.
There are growing fears Yemen is becoming a haven for al Qaeda.
US President Barack Obama says an al Qaeda-linked organisation in Yemen appeared to have trained the Nigerian accused of trying to carry out the attack, Umar Farouk Abdulmutallab. He is being held in a US prison.
The international pressure on Yemen to deal with al Qaeda has produced a quick response, the BBC reports.
Government officials say two militants were killed as security forces moved in on a man believed to be one of the leaders in the movement in Yemen, who they suspect was planning attacks on embassies.
US Secretary of State Hillary Clinton says the Yemeni government has to take steps to restore stability, or risk losing Western support.