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.This August 29, 2017 photo shows an elderly Rohingya refugee asleep with while holding her sleeping grandson in her lap at Kutupalong refugee camp in Ukhiya after crossing the border from Myanmar into Bangladesh (AFP Photo)

A series of coordinated 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 neighbors, but added there were "hundreds and hundreds" of people stuck there.

The treatment of about 1.1 million Muslim Rohingya in Myanmar has become the biggest challenge for leader Aung San Suu Kyi, who has been accused by Western critics of not speaking out on behalf of a minority who have long complained of persecution.

The Rohingya are denied citizenship in Myanmar and regarded as illegal immigrants, despite claiming roots there that go back centuries.