A detainee in Myanmar's Rakhine state died from injuries allegedly sustained in custody, his family said, as the army continues to engage the Arakan Army (AA).

Myanmar's army has deployed thousands of troops to the troubled western state in recent months to try to crush the AA, who say they are fighting for more autonomy for ethnic Rakhine Buddhists.

The military has cut off whole villages and rounded up locals for interrogation as it tries to flush out AA members and sympathisers.

Zaw Win Hlaing, 28, was detained on June 19 in Mrauk U township, his mother told AFP by phone.

Five days later he was sent to a local hospital before being transferred for treatment in the state capital, Sittwe.

But his condition deteriorated and he died on Monday, local media reported.

His mother confirmed Wednesday her son's death, saying he had coughed up "huge amounts of blood".

"My son told me they beat and tortured him by hitting his back with a longyi (a traditional sarong-like garment) packed with stones," Thein Nu Sein said, sobbing.

Army spokesman Brigadier General Zaw Min Tun told AFP he had heard the accusations about Zaw Win Hlaing's death, adding any official complaint would be investigated.

He insisted torture is "banned" during military interrogations.

© AFP