MIT professor Noam Chomsky has warned that an attack on Syria without United Nations support would be a war crime.

“As international support for Obama’s decision to attack Syria has collapsed, along with the credibility of government claims, the administration has fallen back on a standard pretext for war crimes when all else fails: the credibility of the threats of the self-designated policeman of the world,” Chomsky told The Huffington Post in an article published Monday.

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He added that a U.S. strike against Syria without approval from the United Nations would be a “very serious” war crime.

Chomsky said in July that supplying the Syrian rebels with arms would not ultimately resolve the civil war. He advocated for international negotiations.

“This step would most likely produce an escalation of the military conflict and open the door to further military upgrading and expansion on the part of the regime, leading to increased destruction and the regime staying in place for longer,” he told the Middle East Office of the Heinrich Boell Foundation. “The second approach is to go to Geneva with the cooperation of the major powers, including Russia, and force the regime to accept a truce. These are the options we have.”

United Nations inspectors spent about two weeks investigating an alleged poison gas attack that occurred August 21 in the Damascus suburb of Ghouta. The team of inspectors has returned to The Hague in the Netherlands to complete their report on the incident.

The Obama administration, however, has suggested it could take military action before the UN report is completed. Secretary of State John Kerry said Sunday there was ample proof that Assad’s regime used sarin gas against rebels.

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Obama has asked Congress to authorize military strikes against Syria over the use of chemical weapons.