Thousands of Rohingya refugees held "Genocide Remembrance Day" rallies on Thursday across a huge network of camps in Bangladesh, marking five years since fleeing a military offensive in Myanmar.

In August 2017 around 750,000 of the mostly Muslim minority streamed over the border with mainly Buddhist Myanmar to escape the onslaught, which is now the subject of a landmark genocide case at the UN's top court.

Today there are nearly a million Rohingya -- half of them under 18 -- in rickety huts in camps where the mud lanes regularly become rivers of sewage during monsoon rains.

Thousands staged rallies in many of the camps on Thursday, holding banners, shouting slogans and demanding a safe return to their home state of Rakhine in western Myanmar.

"Today is the day thousands of Rohingya were killed," young leader Maung Sawyedollah said with tears in his eyes as he led a rally in Kutupalong -- the world's largest refugee settlement.

"Five years ago this day nearly one million Rohingya were displaced. On this day in 2017 more than 300 of our villages were burnt down to ashes," he said.

"All we want is a safe and dignified return to our homeland," said Sayed Ullah, another community leader.

"Unfortunately, our cries have fallen on deaf ears. The international community is not doing anything," he added.

Other protesters recalled the horror of the Myanmar offensive, which the government says was a legitimate response to militant attacks.

"First they burnt our house. My mother was hiding in the house and they dragged her from there. First they cut her hands and then cut her throat," said Sufia Khatun, 42.

Several attempts at repatriation have failed, with Rohingya refusing to return without guarantees for their security and rights in Myanmar, where many see the minority as foreigners.

Rohingya say that the security situation in the Bangladeshi camps -- surrounded by barbed wire -- is also deteriorating, with at least 100 people killed since 2017.

Much of the violence is blamed on a Rohingya insurgent group and gangs involved in drug smuggling and human trafficking, which find easy recruits among the many bored young men in the camps.

"It's a prison for the Rohingyas," said one young activist, declining to give his name for fear of retaliation from Bangladeshi police.

"Rohingya shops were demolished. We need to take permission to go out of the camps to meet our relatives," he added.

A survey of the refugees published by Doctors Without Borders (MSF) on Thursday showed that the camps were becoming increasingly unhygienic, with soaring rates of diarrhoea and skin infections like scabies.

The UNHCR has called for more funding and support from the international community.

"Generations could be affected if we fail in our obligation to protect the Rohingya and all the people of Myanmar, their fundamental rights and dignity," Noeleen Heyzer, UN special envoy on Myanmar, said Thursday after a visit to the camps.

To ease overcrowding, Bangladesh authorities have relocated about 30,000 Rohingya to an island but there are concerns it is prone to flooding.

"Voluntary and sustainable repatriation is the only solution to the crisis," said Bangladesh Foreign Minister A.K. Abdul Momen.