About 18,000 Rohingya Muslims are estimated to have crossed into Bangladesh in the last week, the International Organization for Migration (IOM) said on Wednesday, seeking to escape the worst violence in Myanmar’s northwest in at least five years.

A series of co-ordinated attacks by Rohingya insurgents on security forces in the north of Myanmar’s Rakhine state on Friday and ensuing clashes triggered the exodus, while the government evacuated thousands of Rakhine Buddhists.

The IOM said it was difficult to estimate the number of people stranded in the no man’s land at the border between the neighbouring countries, but added there were “hundreds and hundreds” of people stuck there. 

A majority of the country’s estimated one million Rohingya live in the northern part of Rakhine state, where Rohingya insurgents launched co-ordinated attacks last week against police posts, setting off allegedly brutal retaliation by government forces.

Human rights groups and advocates for the Rohingya say the army retaliated by burning down villages and shooting civilians. The government blames Rohingya insurgents for the violence, including the arson.

The official death toll in the violence was 96 as of Sunday, and the actual number is likely to be higher in Myanmar, also known as Burma.