UN chief Antonio Guterres has said he heard “unimaginable accounts of killing and rape” from Rohingya refugees who had fled to Bangladesh since last August to escape violence in Burma.

Guterres said after visiting sprawling refugee camps in Bangladesh’s Cox’s Bazar district that the hundreds of thousands of Rohingya Muslims who have taken shelter there want “justice and a safe return home”.

The UN secretary general was visiting Bangladesh to meet refugees who have been seeking protection from the United Nations and the international community.

He said at a news conference that the refugees had to live under terrible conditions in the camps because of massive violations of their human rights in Burma. He praised Bangladesh’s government for being generous towards the refugees.

“It is impossible to visit these camps without breaking our hearts,” Guterres said. “It is possibly one of the most tragic stories in relation to ... systematic violation of human rights.”

On Sunday, he met Bangladeshi prime minister Sheikh Hasina and assured her of the UN’s continuing support for Rohingya.

The recent wave of violence in Burma began when Rohingya insurgents staged a series of attacks on August 25 on about 30 security outposts and other targets.

In a subsequent crackdown described by UN officials as “ethnic cleansing”, Burmese security forces have been accused of rape, killing, torture and the burning of Rohingya homes. Thousands are believed to have been killed.

Rohingya are denied citizenship in overwhelmingly Buddhist Burma, where they have faced persecution for decades. They are derided as “Bengalis”, and many in Burma believe they are illegal migrants from Bangladesh.

Bangladesh and Burma signed an agreement in November to start repatriating the Rohingya in January, but the process has been delayed over safety concerns and a complicated verification process.

Global human rights groups and the UN said conditions in Burma were not safe for the refugees to return.