Humanitarian organisations helping Rohingya Muslim refugees in Bangladesh say they need $434m (£327m) over the next six months to help up to 1.2 million people, many of them children, who need life-saving help.

There are an estimated 809,000 Rohingya sheltering in Bangladesh after fleeing violence and persecution in Myanmar, more than half a million of whom have arrived since 25 August to join 300,000 Rohingya who were already in Bangladesh.

“The Rohingya population in Cox’s Bazar is highly vulnerable, many having experienced severe trauma, and are now living in extremely difficult conditions,” Robert Watkins, the UNs resident coordinator in Bangladesh, said, referring to the Bangladeshi border district where most Rohingya are living.

Bangladesh and humanitarian organisations are struggling to help the 509,00 Rohingya who have arrived since 25 August, when attacks by Rohingya militants triggered a Myanmar military offensive that the UN has described as ethnic cleansing.

Myanmar rejects accusations of ethnic cleansing. It says its security forces are fighting insurgents of the Arakan Rohingya Salvation Army, who claimed responsibility for attacks on about 30 police posts and an army camp on 25 August.

The insurgents were also behind similar but smaller attacks in October last year that also led to a brutal Myanmar army response, triggering the flight of 87,000 Rohingya to Bangladesh.

On Wednesday, the UN committees for women and children’s rights called on Myanmar’s authorities to immediately stop violence in northern Rakhine state.

“We are particularly worried about the fate of Rohingya women and children subject to serious violations of their human rights, including killings, rape and forced displacement,” the committees on the elimination of discrimination against women and on the rights of the child said.

“Such violations may amount to crimes against humanity and we are deeply concerned at the state’s failure to put an end to these shocking human rights violations being committed at the behest of the military and other security forces.”

The agencies’ aid plan factors in the possibility of another 91,000 refugees arriving, as the influx was continuing day to day, Watkins said.

“The plan targets 1.2 million people, including all Rohingya refugees and 300,000 Bangladeshi host communities over the next six months,” he said.

“A rapid response from donors to this plan is essential if the humanitarian organisations are to move ahead with critical activities to save lives, and provide protection to Rohingya refugees in Bangladesh.”