WASHINGTON — The US Supreme Court was urged Tuesday to help pin down the definition of "providing material support to terrorism," a charge widely used in the United States since the September 11, 2001 attacks.

In a case brought by both the government and a rights group, the Humanitarian Law Project, the nine justices were called upon to define what kind of support can be included in such a broad allegation.

The justices however appeared to be just as divided as the lawyers for both sides.

An appeals court has already ruled that providing "personnel" could be used to bring such charges, whereas "training" and "services" were considered too vague. On a fourth term included in the charge of providing "expert advice or assistance," the appeals court returned a split decision.

The Project has sought to help groups such as the Kurdistan Workers' Party (PKK) and the Tamil Tigers (LTTE) -- both designated by Washington as terror groups -- in peaceful conflict resolution and human rights issues.

"The government spent the last 10 years arguing our client cannot advocate for peace," said David Cole, lawyer for the Project, which risks facing charges if it continues to provide such help.

The group, which has been seeking a clarification in the law for the past decade to continue its work, has won the support of former US president Jimmy Carter, who has said fighting violence at times means talking to the perpetrators.

Cole argued that the groups the Project wants to help have nothing to do with Al-Qaeda, responsible for the 2001 terror attacks in the United States. He also said his group only provides help with the legal activities its clients.

Created in 1996, the charge of "providing material support to terrorism" was strengthened under the Patriot Act, adopted after the 2001 attacks and then amended in 2004.

About 150 people have faced such charges -- often for directly providing funds or arms to terror organizations or for helping in planning attacks -- of which 60 have been sentenced, some to life imprisonment.

It has been a key tool in the war on terror, and has been leveled against many Guantanamo Bay detainees.

And in May, two leaders of what was once the largest Muslim charity in the United States were sentenced to 65 years in jail for supporting Palestinian militants when they were found guilty on 108 charges of providing material support to terrorists, money laundering and tax fraud.

US solicitor general Elena Kagan argued that any help to outlawed organizations will only reinforce their terrorist activities.

And even if there were rare exceptions, that "would be a thimbleful in comparison" to "the ocean of legitimate cases" where the charges would apply.

The nine justices, who are due to rule by June, peppered the two sides with questions. But judge Antonin Scalia seemed to have already formed an opinion.

"Any assistance you provide to these organizations cannot be separated from their terrorist activities," he told the hearing.