The Department of Justice has taken the rare step of seeking to strip a convicted militant of his US citizenship as he serves the last several years of a 20-year prison sentence for plotting to destroy New York's Brooklyn Bridge.

Some national security experts suggested Tuesday the move might signal a new, tougher line under President Donald Trump.

The case involves Iyman Faris, 47 and born in Pakistan, who was sentenced in 2003 for aiding and abetting the Al Qaeda terrorist group with his plan to cut through cables that support the iconic bridge. At the time, it was among the highest profile terrorism cases in the aftermath of the September 11, 2001, attacks that killed nearly 3,000 people.

A 17-page filing Monday in US District Court in southern Illinois where Faris is imprisoned launched a revocation process that is likely to take years.

The court filing argues that Faris lied on immigration papers before becoming a naturalised US citizen in 1999 and that his terrorist affiliations demonstrated a lack of commitment to the US Constitution.

The acting assistant attorney of the Justice Department's civil division, Chad Readler, vowed in a statement about the filing to “continue to pursue denaturalisation proceedings against known or suspected terrorists who procured their citizenship by fraud”. He added: “The US government is dedicated to ... preventing the exploitation of our nation's immigration system by those who would do harm to our country.”

Faris, known as Mohammad Rauf before becoming a US citizen and who once worked as a truck driver in Ohio, is scheduled for release from the US Penitentiary at Marion, Illinois, on December 23, 2020, according to the US Bureau of Prisons.

Karen Greenberg, director of the Fordham Law School's Centre on National Security in New York, said the federal government has been aggressive in previous decades about revoking the citizenship of accused Nazis living in the US. But she says it's largely unheard of for revocation proceedings to be launched against naturalised US citizens imprisoned for terrorism.

Most Americans would almost certainly back steps to strip citizenship from someone like Faris.

Prosecutors have also accused him of meeting with Osama bin Laden in 2000 and alleged that the planned attack on the bridge could have been designed to be part of a second wave of attacks to follow those on 9/11.

But Greenberg said making the revocation of a terrorist's US citizenship established policy would only add to a trend since 9/11 of treating accused terrorists differently than other suspects.

Stripping someone's citizenship, she said, also appeared to be a way of adding on extra punishment not in the criminal statute itself.

“Why isn't it enough that we put him in prison and give him the sentence he was given?” she said.

She added that the effort against Faris could be seen as another example of how the Trump administration “tinkers with the established way we do things”.

The government can't strip US-born citizens of their citizenship, so can't use the revocation of citizenship as a tool to punish attackers born and raised in the US.-AP