The largely secret criteria for the government's no-fly list have been upheld by the 9th US Circuit Court of Appeals in a case involving four people who sued over being barred from flying.

A three-judge panel ruled unanimously Monday that the government has gone as far as the law requires in explaining the listing of each plaintiff without breaching national security, the San Francisco Chronicle reported.

"The government has taken reasonable measures to ensure basic fairness to the plaintiffs, and followed procedures reasonably designed to protect against erroneous deprivation of the plaintiffs' liberty," Judge Raymond Fisher wrote.

Freedom to travel is important but "it must be balanced against the government's urgent interest in combating terrorism," he said.

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The ruling was criticised by the American Civil Liberties Union, which represented the plaintiffs.

"Our clients have been unable to visit family, pursue job opportunities or fulfill religious obligations for over nine years based on vague criteria, secret evidence and unreliable government predictions," said ACLU attorney Hina Shamsi.

The no-fly list has kept tens of thousands of people from boarding commercial aircraft flying to, from or over the U.S. since 2001.

The Transportation Safety Administration kept virtually all aspects of the list secret until 2015, when the 9th Circuit ruled that people were entitled to learn whether they were on the list and a summary of the reasons, without revealing classified information.

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The four plaintiffs in the lawsuit are US citizens with no criminal records who have been barred from flying for nine years or more.

They claimed federal officials offered only vague reasons for why they are on the list.

Information filed to the court by the government stated that one man was on the list because of "concerns" about his travel to Yemen in 2010, the Chronicle reported. Another man allegedly told the FBI he distributed speeches by a now-deceased terrorist, and a third allegedly admitted he engaged in militant acts in Somalia but he contended the admissions were coerced through interrogation.

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Judge Fisher wrote that the court examined the government's full explanations and concluded that "an unclassified summary of the undisclosed reasons was not possible."

Each plaintiff had "fair notice" that their alleged conduct, such as traveling abroad for weapons training with organisations linked to terrorism, "would raise suspicion" under the published standards, Fisher said.