A planned visit by a United Nations team to Rakhine state in Myanmar, the region which almost half a million Rohingya Muslims have fled in recent months claiming violent persecution, was cancelled on Thursday by Myanmar's government, U.N. spokesman Stephane Dujarric confirmed to CBS News.

CBS News' Pamela Falk reports that a U.N. Security Council meeting has been called for Thursday afternoon with a briefing by Secretary-General António Guterres on the crisis in Myanmar.

The United Nations team was to be the first group of international observers allowed by Myanmar's government into Rakhine state since the violence there escalated rapidly in August. According to the U.N., Myanmar's government gave no explanation for the sudden cancellation of the planned visit.

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An ethnic Rohingya insurgent group attacked police posts in Myanmar, sparking a brutal crackdown by the country's military and security forces, which have been accused of razing scores of villages in what U.N. officials have said amounts to "ethnic cleansing."

An estimated 480,000 Rohingya have fled to neighboring Bangladesh since late August alone, and there were already significant numbers seeing refuge there before as the crisis has been simmering for years.

The head of the U.N.'s migration agency warned Wednesday about increasing reports of sexual violence directed at Rohingyas in the increasingly-over-crowded Bangladesh refugee camps.

Director-General William Lacy Swing of the International Organization for Migration said he was "shocked and concerned" about the reports of sexual and gender-based violence among Rohingya newly arrived in Cox's Bazar, Bangladesh.

IOM said rape, sexual assault, domestic violence, child marriage and other forms of gender-based violence have been identified. It did not specify who was responsible for the violence.

The agency is coordinating the humanitarian response among U.N. agencies and aid providers amid the exodus of people who have reached Cox's Bazar since Aug. 25.

An agency statement said IOM doctors have treated dozens of women who experienced "violent sexual assault" since August, but that the known cases likely represent only a "small portion" of actual cases.

Swing said such "egregious violence and abuse is underreported" even in more stable and better-resourced humanitarian aid situations.

"Sexual and gender-based violence is a severe, life-threatening public health and human rights abuse," he said. "Particularly women and girls, but also men and boys, have been targeted for and are at risk of further exploitation, violence and abuse simply because of their gender, age and status in society."