WASHINGTON — The Supreme Court has upheld a federal law that bars “material support” to foreign terrorist organizations, rejecting a free speech challenge from humanitarian aid groups. The court ruled 6-3 Monday that the government may prohibit all forms of aid to designated terrorist groups, even if the support consists of training and advice about entirely peaceful and legal activities. Material support intended even for benign purposes can help a terrorist group in other ways, Chief Justice John Roberts said in his majority opinion. “Such support frees up other resources within the organization that may be put to violent ends,” Roberts said. Justice Stephen Breyer took the unusual step of reading his dissent aloud in the courtroom. Breyer said he rejects the majority’s conclusion “that the Constitution permits the government to prosecute the plaintiffs criminally” for providing instruction and advice about the terror groups’ lawful political objectives. Justices Ruth Bader Ginsburg and Sonia Sotomayor joined the dissent. 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 terror groups. Only a handful dealt with the kind of speech involved in the case decided Monday. The aid groups involved had trained a group 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 Turkish outfit a terrorist organization in 1997. They also wanted give similar help to a group in Sri Lanka, but it, too, was designated a terrorist organization by the U.S. in 1997. Nearly four dozen organizations are on the State Department list, including al-Qaida, Hamas, Hezbollah, Basque separatists in Spain and Maoist rebels in Peru. The humanitarian groups, including the Humanitarian Law Project; Ralph Fertig, a civil rights lawyer; and Dr. Nagalingam Jeyalingam, a physician, want to offer assistance to the Kurdistan Workers’ Party in Turkey or the Liberation Tigers of Tamil Eelam in Sri Lanka. The government says the Kurdish rebel group, known as the PKK, has been involved in a violent insurgency that has claimed 22,000 lives. The Tamil Tigers waged a civil war for more than 30 years before their defeat last year.