Bangladesh again warned Myanmar on Friday of violating its airspace as violence has driven nearly 400,000 ethnic Rohingya Muslims into Bangladesh over the past three weeks in what the UN has described as "ethnic cleansing."

The Foreign Ministry in Dhaka said Myanmar military drones and helicopters had violated its airspace on September 10, 12 and 14, as well as on August 25.

The ministry said Myanmar's "provocations" could lead to "unwarranted consequences."

Ties between Bangladesh and Myanmar have been strained since Myanmar's military launched "clearance operations" on August 25 following an attack by Rohingya militants on police posts.

Read more: Myanmar's Rohingya rebels: What you need to know

Burning villages

Human rights organizations say Myanmar's military and Buddhist vigilante groups have burned villages and killed villagers as part of a scorched earth policy to drive them out of the country. 

"Our field research backs what the satellite imagery has indicated — that the Burmese [Myanmar] military is directly responsible for the mass burning of Rohingya villages in northern Rakhine State," said Phil Robertson, deputy Asia director at Human Rights Watch.

Myanmar has denied the allegations. 

The UN says nearly 400,000 Rohingya have been forced across the border in Bangladesh, raising concerns of a humanitarian crisis.

"There's really no sign that this flow of people is going to dry up," said Chris Lom of the International Organization for Migration.

Read more: Myanmar's Rohingya: A history of forced exoduses

Humanitarian crisis

Aid agencies are still blocked from accessing all of Rakhine state due to what the Myanmar military says are security concerns.

The violence has mounted international pressure on Myanmar's military leaders and civilian administration, led by Nobel laureate Aung San Suu Kyi.

US State Department official Patrick Murphy was set to visit Myanmar to press for an end to violence and allowing humanitarian aid in Rakhine state.

But Myanmar officials said Friday Murphy would not be allowed to access the northern part of Rakhine state at the center of the violence during his trip that includes a visit to the state capital, Sittwe.

The UN refugee agency said Friday it was appealing for $30 million (25 million euros) for the emergency humanitarian aid for Rohingya in Bangladesh. Myanmar says refugees can return if they are citizens, but most Rohingya are stateless despite having lived in Myanmar for generations.

cw/cmk (dpa, Reuters)