WASHINGTON - The Supreme Court upheld the government's authority Monday to ban aid to designated terrorist groups, even when that support is intended to steer the groups toward peaceful and legal activities.

The court left intact a federal law that the Obama administration considers an important tool against terrorism. But human rights organizations say the law's ban on providing training and advice to nearly four dozen organizations on a State Department list squanders a chance to persuade people to renounce extremism.

The justices voted 6-3 to reject a free-speech challenge from humanitarian aid groups to the law that bars "material support" - everything from money to technical know-how to legal advice - to foreign terrorist organizations.

The aid groups were only challenging provisions that put them at risk of being prosecuted for talking to terrorist organizations about nonviolent activities.

But Chief Justice John Roberts said in his opinion for the court that material support intended even for benign purposes can help a terrorist group in other ways.

"Such support frees up other resources within the organization that may be put to violent ends," Roberts said in an opinion joined by four other conservative justices, but also the liberal Justice John Paul Stevens.

The court often looks skeptically on laws that criminalize speech and holds them to a high level of scrutiny. But Roberts said there is good reason in this case to defer to Congress and the president, "uniquely positioned to make principled distinctions between activities that will further terrorist conduct and undermine United States foreign policy, and those that will not."

Justice Stephen Breyer took the unusual step of reading his dissent aloud in the courtroom. "Not even the 'serious and deadly problem' of international terrorism can require automatic forfeiture of First Amendment rights," he said. Justices Ruth Bader Ginsburg and Sonia Sotomayor joined the dissent.

Solicitor General Elena Kagan argued the government's case at the high court more than two months ago before President Barack Obama nominated her to replace Stevens, who will retire in days.

The Obama administration said the "material support" law is one of its most important terror-fighting tools. It has been used about 150 times since Sept. 11, resulting in 75 convictions. Most of those cases involved money and other substantial support for terrorist groups.

Only a handful dealt with the kind of speech involved in the case decided Monday.

Human rights groups said they were stunned by the ruling.

David Cole, a Georgetown law professor who represented the aid groups at the Supreme Court, said the court essentially ruled that "the First Amendment permits the government to make human rights advocacy and peacemaking a crime."

The aid groups involved had trained the Kurdistan Workers' Party in Turkey on how to bring human rights complaints to the United Nations and assisted them in peace negotiations, but suspended the activities when the U.S. designated the Kurdish organization, known as the PKK, a terrorist group in 1997. They also wanted to give similar help to the Liberation Tigers of Tamil Eelam in Sri Lanka, but they, too, were designated a terrorist organization by the U.S. in 1997.

Representatives of the Tamil Tigers appealed the designation to a federal appeals court, which upheld the government. The PKK has not challenged its terrorist designation.

No serious dispute exists in the U.S. over the designation for groups such as al-Qaida, Abu Nidal and the Shining Path. But some others have legitimate political arms and extensive social missions as well as associations with violence through paramilitary or insurgent means. Hamas, for example, won a majority of Palestinian support in democratic elections.

Once the State Department places a group on the list, it is illegal for Americans or others in the country to provide "material support or resources" to the group. The law also bars travel to the U.S. by representatives or members of the group and freezes any assets that it has in U.S. jurisdictions.