Archbishop Desmond Tutu is breaking his vow of silence in retirement to call on Aung San Suu Kyi to end her country's persecution of the Rohingya Muslim minority group.

In a heartfelt open letter to Myanmar's de facto leader, the 85-year-old described her as a 'dearly beloved sister' but said the 'unfolding horror' and 'ethnic cleansing' ongoing in the country's Rakhine State forced him to speak out.

Tutu, a Nobel peace prize winner, urged his fellow laureate to intervene, despite her recent defence of her military's actions.

'I am now elderly, decrepit and formally retired, but breaking my vow to remain silent on public affairs out of profound sadness,' he wrote.

'For years I had a photograph of you on my desk to remind me of the injustice and sacrifice you endured out of your love and commitment for Myanmar's people. You symbolised righteousness,' he said, posting the letter on social media.

'Your emergence into public life allayed our concerns about violence being perpetrated against members of the Rohingya. But what some have called "ethnic cleansing" and others "a slow genocide" has persisted – and recently accelerated.'

Joining the growing list of world leaders calling on Suu Kyi to act, Tutu went on: 'If the political price of your ascension to the highest office in Myanmar is your silence, the price is surely too steep.'

He wrote: 'It is incongruous for a symbol of righteousness to lead such a country. It is adding to our pain.'

Tutu's unexpected intervention comes after another peace prize winner, Malala Yousafzai, told Aung San Suu Kyi 'the world is waiting' for her response.

'Every time I see the news, my heart breaks,' she wrote on Twitter. 'Over the last several years, I have repeatedly condemned this tragic and shameful treatment. I am still waiting for my fellow Nobel laureate Aung San Suu Kyi to do the same.'

Despite the pressure Aung San Suu Kyi defended her military's action in a phone call with Turkish president Recep Tayip Erdogan. She said 'huge iceberg of misinformation' was supporting terrorists in the conflict and insisted the army was protecting everyone in the conflict ridden Rakhine State.

'It is a little unreasonable to expect us to solve the issue in 18 months,' she later told the Delhi-based network Asian News International. 'The situation in Rakhine has been such since many decades. It goes back to pre-colonial times.'