KUTUPALONG CAMP, Bangladesh (Reuters) - Aid agencies have to step up operations “massively” in response to the arrival in Bangladesh of about 400,000 refugees fleeing violence in Myanmar, and the amount of money needed to help them has risen sharply, a senior U.N. official said on Wednesday.

Rohingya refugees wait for the food to be distributed by local organisations near Balukhali makeshift refugee camp in Cox's Bazar, Bangladesh, September 13, 2017. REUTERS/Danish Siddiqui

The exodus of Muslim Rohingya to Bangladesh began on Aug. 25 after Rohingya militants attacked about 30 police posts and an army camp. The attacks triggered a sweeping military counter-offensive by security forces in Buddhist-majority Myanmar which the U.N. rights agency said was a “textbook example of ethnic cleansing”.

“We will all have to ramp up our response massively, from food to shelter,” George William Okoth-Obbo, assistant high commissioner for operations at the U.N. refugee agency, told Reuters during a visit to the Kutupalong refugee camp in Bangladesh.

The United Nations said on Tuesday 370,000 people had crossed into Bangladesh but Okoth-Obbo estimated the figure was now 400,000. He declined to speculate on how many more might come.

Bangladesh was already home to about 400,000 Rohingya, who fled earlier conflict in Myanmar including a similar security crackdown in western Myanmar’s Rakhine state in response to militant attacks in October.

Many of the new arrivals are hungry and sick, without shelter or clean water in the middle of the rainy season.

“We have an emergency within an emergency with conditions in existing camps,” he said, pointing to a mud-clogged road in the camp.

Last week, the United Nations appealed for $77 million to cope with the crisis but Okoth-Obbo said that would not now be enough.

“The appeal that was issued of $77 million on behalf of the aid agencies was based on the situation as it was roughly about two weeks ago,” he said.

“There were only 100,000 people then. We are already four times that figure now. The funds need clearly is going to continue.”

He declined to say how much he thought was needed.

He also declined to say if he thought aid agencies were getting proper access to the conflict zone in Myanmar, though he said it was important to ensure that people were safe where they were.

“Of course, also that access is provided to all the responders to provide humanitarian assistance,” he added.

Myanmar has restricted most aid agency access to the north of Rakhine. Some officials have accused aid agencies of supporting the insurgents.

Okoth-Obbo said he agreed with the Bangladeshi position that the most important solution was for the refugees to be able to return home in safety.

Bangladeshi Prime Minister Sheikh Hasina said on Tuesday the refugees would all have to go home and Myanmar should set up safe zones to enable them to do so.

“Under difficult circumstances this country has kept its borders open,” Okoth-Obbo said of Bangladesh.

“All of us should support that and ensure that the response is strong.”

(Story corrects paragraph 2 reference to ethnic cleansing, not genocide.)