Myanmar's military has launched an internal probe into the conduct of soldiers during a counteroffensive that has sent more than half a million Rohingya Muslims fleeing to Bangladesh, many saying they witnessed killings, rape and arson by troops.

Coordinated Rohingya insurgent attacks on 30 security posts on Aug. 25 sparked a ferocious military response in the Muslim-majority northern part of Rakhine state that the United Nations has said was ethnic cleansing.

A committee led by military Lieutenant-General Aye Win has begun an investigation into the behaviour of military personnel, the office of the commander in chief said on Friday, insisting the operation was justified under Buddhist-majority Myanmar's constitution.

According to a statement posted on Senior General Min Aung Hlaing's Facebook page, the panel will ask, "Did they follow the military code of conduct? Did they exactly follow the command during the operation? After that (the committee) will release full information."

Myanmar is refusing entry to a U.N. panel that was tasked with investigating allegations of abuses after a smaller military counteroffensive launched in October 2016.

But domestic investigations - including a previous internal military probe - have largely dismissed refugees' claims of abuses committed during security forces' so-called "clearance operations".

Thousands of refugees have continued to arrived cross the Naf river separating Myanmar's Rakhine state and Bangladesh in recent days, even though Myanmar insists military operations ceased on Sept. 5.

Aid agencies now estimate that 536,000 people have now arrived in Cox's Bazar district, straining scarce resources of aid groups and local communities.

About 200,000 Rohingya were already in Bangladesh after fleeing persecution in Myanmar, where they have long been denied citizenship and faced restrictions on their movements and access to basic services.

Myanmar's de facto leader, Aung San Suu Kyi, has pledged accountability for human rights abuses and says Myanmar will accept back refugees who can prove they were residents of Myanmar.

The powerful army chief has taken a harder stance, however, telling the U.S. ambassador in Myanmar earlier this week that the exodus of Rohingya - who he said were non-native "Bengalis" - was exaggerated.

In comments to Japan's ambassador carried in state media on Friday, Min Aung Hlaing denied ethnic cleansing was taking place on the grounds that photos showed Muslims "departing calmly rather than fleeing in terror".