President Barack Obama has appealed for calm on the streets of Missouri after days of clashes between police and protesters.
The tensions follow the fatal shooting of a black teenager by a police officer in the St. Louis suburb of Ferguson.
President Obama said there was "no excuse" for "police to use excessive force against peaceful protesters" or for "violence against police".
ITV News' Washington Correspondent Robert Moore reports:
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The decision represents something of a victory for the president who will now be able to enforce most of his controversial 90-day ban.
Cool in the north, warmer air to the south
Police have warned the drug, known locally as "pink champagne", could be deadly.