The US has announced that it will begin profiling US-bound passengers to determine who should get extra screening.

美国宣布将会为所有进出美国的乘客建立一个文件系统以决定对某些嫌疑分子进行额外安全检查。

The rules apply to all travellers heading to the United States

The measures will replace mandatory¹（强制的，命令的） enhanced screening of all travellers from 14 nations, brought in after the failed attack on a flight in December.

Travellers will be picked out according to how closely they match intelligence on potential terrorist threats.

The new strategy results from a review ordered by President Barack Obama.

'Intelligence-led'

Department of Homeland Security Secretary Janet Napolitano said the new measures would use "real-time, threat-based intelligence along with multiple, random² layers of security, both seen and unseen, to more effectively mitigate³（减轻，缓和） evolving terrorist threats".

The US began notifying air carriers on Thursday and the new protocols⁴（协议，条款） are being implemented⁵（执行，实施） immediately, a senior administration official told the BBC.

The New York Times says the new strategy will more precisely⁶ target those individuals about whom security officials have concerns.

"This is not a system that can be called profiling in the traditional sense. It is intelligence-based," the newspaper quoted an official as saying.

Passengers were told to expect increased use of explosives trace detection, advanced imaging technology, sniffer dogs（嗅探犬） and pat downs（拍身检查） .

The measures will apply to US citizens, as well as foreigners travelling to America.

Congressman⁷ Peter King, the top Republican on the homeland security oversight⁸ committee, said making better use of intelligence was "a step forward".

'No fly' list

The US government currently has a 6,000-name "no fly" list of suspected terrorists, who are banned from flights to or within US territory.

This will be supplemented（增补，补充） by cross-referenced information that may see passengers subject to further screening even if their names are not flagged, the Wall Street Journal reported.

The screening will take into account（重视，考虑） characteristics like nationality, age, recently visited countries and partial names, the newspaper said.

While the US does not have the authority to screen passengers in foreign airports, it can sanction（制裁，处罚） air carriers if they do not agree to follow US guidelines for international aviation（航空，飞行） security.

A 23-year-old Nigerian man - Umar Farouk Abdulmutallab - has been charged with attempting to destroy a plane after he allegedly tried to detonate（引爆，爆炸） a bomb on a passenger jet arriving in the US on 25 December 2009.

He was overpowered by passengers and crew shortly before the Northwest Airlines plane landed in Detroit from Amsterdam.

In the wake of that incident new rules were brought in requiring extra screening for passengers from, or travelling through, Afghanistan, Algeria, Cuba, Iran, Iraq, Lebanon, Libya, Nigeria, Pakistan, Saudi Arabia, Somalia, Sudan, Syria and Yemen.

The BBC's Adam Brookes in Washington says those hastily-imposed rules will now be replaced.

The specifics of these new measures remain mysterious, but they may involve elements like analysing an individual's travel record and matching it against current intelligence, all before that individual boards a plane.

Reportedly the US government will give more information to airlines and foreign security staff to use at check-in, and officials said passengers flying to the US might notice more random screening, more checks for traces of explosives and more use of advanced imaging technology.