United Nations (UN) Secretary General Antonio Guterres is concerned about India's plans to deport Rohingya Muslim refugees it says are living in the country illegally, his spokesperson said on Monday.

Tens of thousands of Rohingya have fled persecution in Buddhist-majority Myanmar to neighbouring Bangladesh since the early 1990s, with some of them then crossing over a porous border into Hindu-majority India.

“Obviously, we have our concerns about the treatment of refugees," Guterres' Deputy Spokesman Farhan Haq told reporters at the regular briefing at UN Headquarters in New York. "Once refugees are registered, they are not to be returned back to countries where they fear persecution.”

He was responding to a question about India's announcement that it would deport Rohingya Muslims regardless of whether they are registered as UN refugees or not.

Guterres, who was the UN High Commissioner for Refugees (UNHCR) before his appointment as UN chief, is a staunch supporter of the cause of refugees.

The spokesman said that the UNHCR office would take up the issue with the Indian government. He reminded India of UN's position against deporting refugees.

“You are aware of our principle of non-refoulement,” he said, referring to the doctrine in the UN Convention on the Status of Refugees.

"According to the principle, refugees cannot be returned to a place where their life or freedom would be threatened on account of their race, religion, nationality, membership of a particular social group or political opinion," he said.

Amnesty International has said deporting and abandoning the Rohingya would be “unconscionable”.