That incident came a day after Saudi Arabia joined the US and Brazil in abstaining from a UN General Assembly vote that overwhelmingly endorsed an international ban on cluster bomb use.
"Brazil should be on notice that its rockets are being used in unlawful attacks in the Yemeni war," said HRW arms director Steve Goose.
The weapons can contain dozens of smaller bomblets that disperse over large areas, often continuing to kill and maim civilians long after they are dropped.
The Saudi-led coalition this week said it had made "limited use" of British-made cluster bombs, a type of weapon which 100 countries have already pledged not to use.
The alliance, which intervened in support of Yemen's government in March 2015 after the Huthis overran much of the country's northern and central regions, has come under repeated criticism over civilian casualties.
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