Dwindling humanitarian aid for Rohingya refugees threatens to exacerbate one of the world's largest humanitarian crises, the UN's refugee chief said on Tuesday.

Bangladesh is home to around a million members of the mostly stateless minority, many of whom fled a Myanmar military crackdown in 2017 that is now subject to a genocide probe at the International Criminal Court.

Frustration is widespread over the lack of progress in a repatriation deal, rampant lawlessness in the refugee settlements and cuts to international humanitarian aid.

Humanitarian assistance "is declining" amid crises in Ethiopia, Sudan, Afghanistan, Ukraine and Israel, Filippo Grandi told reporters on the sidelines of a regional meeting.

"This is a crisis that should not be forgotten... If contributions decline, we are in trouble."

The Rohingya crisis had only 42 per cent of the $875.9 million funding needed this year, he said.

Budget cuts have forced the UN World Food Programme to steeply reduce humanitarian aid to the Rohingya camps this year, with funding for rations now at $8 per refugee per month.

Malnutrition in the sprawling camps is already rampant, rights groups say.

The United Kingdom's Minister for the Indo-Pacific Anne-Marie Trevelyan announced a 4.5 million pound ($5.4 million) funding contribution on Tuesday.

Grandi said repatriation to Myanmar was still "Plan A", even as he acknowledged many Rohingya were still afraid to return to the country where they are widely viewed as interlopers from Bangladesh.

Bangladesh and Myanmar are working on a pilot programme to begin repatriating a limited number of Rohingya, despite concerns from rights groups who say conditions are not safe for their return.

Rohingya in Myanmar are denied citizenship and access to healthcare and require permission to travel outside of their townships.

Myanmar junta chief Min Aung Hlaing -- who was head of the army during the crackdown -- has dismissed the term Rohingya as "imaginary".

"Plan B is the status quo, people continue to be in host countries... but this is not sustainable in the long term," Grandi said.