WASHINGTON - THE US government is compiling electronic files on the travel habits of millions of Americans who take trips overseas, The Washington Post reported on Saturday.

Citing documents obtained by a civil liberties group and statements by unnamed government officials, the newspaper said the retained data included travel companions, persons with whom Americans plan to stay abroad, the personal items they carry during their journeys, and even the books that travellers have carried.

The personal travel records are intended to be stored for as long as 15 years as part of the Department of Homeland Security's effort to assess the security threat posed by all travelers entering the country, the report said.

Officials say the records, which are analysed by the department's Automated Targeting System, help border officials distinguish potential terrorists from innocent people entering the country, The Post said.

But new details about the information being retained suggest that the government is monitoring the personal habits of travellers more closely than it has previously acknowledged, the report said.

The Automated Targeting System has been used to screen passengers since the mid-1990s, but the collection of data for it has been greatly expanded and automated since 2002, according to the paper.

Officials defended the retention of highly personal data on travelers not involved in or linked to any violations of the law, the report said.

But civil liberties advocates have alleged that the type of information preserved by the department raises alarms about the government's ability to intrude into the lives of ordinary people, The Post pointed out.

The millions of travellers whose records are kept by the government are generally unaware of what their records say, and the government has not created an effective mechanism for reviewing the data and correcting any errors, the paper said.

The activists alleged that the data collection effort, as carried out now, violates the Privacy Act, which bars the gathering of data related to Americans' exercise of their First Amendment rights, such as their choice of reading material or persons with whom to associate, according to the report.