Local forces killed ethnic minority Kaman Muslims in the town of Kyauk Pyu while government troops "stood by and watched", according to New York-based Human Rights Watch (HRW).
Soldiers from the local border guard force meanwhile "severely beat" dozens of displaced Rohingya Muslims arriving by boat near the Rakhine state capital Sittwe from violence-hit local villages, it said.
Elsewhere, however, security forces provided protection to displaced Rohingya and Kaman Muslims by firing shots in the air to fend off Buddhist mobs and by providing water and food, according to HRW.
Two major outbreaks of violence between Muslims and Buddhists in Rakhine since June have left 180 people dead and more than 110,000 displaced. Most of the displaced were Rohingya, a minority group that has faced decades of discrimination.
HRW released new satellite imagery which it said showed extensive destruction of homes and other property in three mainly Rohingya areas.
It reported accounts of "gruesome casualties" on both sides of the Buddhist-Muslim clashes, including beheadings and killings of women and children.
"The satellite images and eyewitness accounts reveal that local mobs, at times with official support, sought to finish the job of removing Rohingya from these areas," said Brad Adams, the watchdog's Asia director.
He urged US President Barack Obama to press Myanmar's reformist leader Thein Sein on the issue when he makes a historic visit to Yangon on Monday following sweeping political changes in the former pariah state.
"The absence of accountability for this horrific violence gives a green light to extremists to continue their attacks and abuses," Adams said.
After the initial eruption of violence in June, HRW accused security forces of opening fire on Rohingya, committing rape and standing by as rival mobs attacked each other.
Myanmar's 800,000 Rohingya are seen as illegal immigrants from neighbouring Bangladesh by the government and many Burmese. They have long been considered by the United Nations as one of the most persecuted minorities on the planet.
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@Linchpin: But still we should not poke into others matter. Why do feel this our responsibility whenever Muslims get persecuted in other countries. In fact we should support ET's earlier stories, especially, by Ayesha Siddiqa, which made it clear that situation in Burma is no as bad as depicted by Islamists on the social media. Islamists even shared fake photos which tell a lot about the issue and ET is the only resource which brought the other side of the story. We should learn lesson from India which is growing rapidly because they are not poking in others nose.
@VINOD: This article is written by Rueters has nothing to do with pakistan - you seem obsessed with Pakistan "Why is vinod more worried about the affairs of others? It sounds Funny when you talk about Pakistan when your not worried about the suicide of 250000 farmers or 8 million female babies that have been aborted in your country.
Why Pakistan is more worried about the affairs of others? It sounds funny when you talk about Muslims of Kashmir or Palestine or Myanmar; when you are not worried about thousands of Pakistanis rotting in the camps at Bangladesh from last 41 years. When you are not worried about thousands of Muslims being killed in Pakistan itself in sectarian violence.