Rohingya are being charged up to the equivalent of $US250($319) each to make the short boat journey from Rakhine state to the Bangladesh border, a fortune for Myanmar's poorest people, authorities in Bangladesh say.

Some boats are also smuggling methamphetamine from Myanmar, one of the world's largest producers of the illicit drug, authorities say.

"The boats are turning to carry passengers they are not supposed to," Lieutenant Colonel Ariful Islam from Border Guards Bangladesh told Reuters.

Bangladesh police said they rescued 20 Rohingya this week, including seven women and five children, from a border village where they were being held by people smugglers.

"They were being held there by a gang of boat owners and crew who demanded $US250 per person for the two-hour boat ride from Myanmar," said Ruhul Amin from Bangladesh's Rapid Action Battalion, who led the rescue mission.

The boat ride normally costs only a few dollars.

Since the exodus began police have freed almost 2000 people in raids on coastal villages, including about 1000 Rohingya who were being held in six houses, Major Amin told Agence France Presse.

Bangladeshi authorities have destroyed 20 boats that ferried Rohingya from Rakhine in the past few days.

Experts expect many of more than 500,000 Rohingya who have fled in the past month to be recruited by people-smugglers and be taken to the sea in unsafe boats.

Most of the refuges want to reach Thailand, Malaysia, Indonesia and Australia. The first boats are expected to leave at the end of the monsoon season in the coming weeks.

Even before the huge Rohingya exodus after fresh violence erupted in Rakhine in late August, hundreds of thousands of refugees living in squalid Bangladesh camps had been fertile recruiting grounds for smugglers.

Thai authorities cracked down on people-smugglers using sea routes in 2015 when mass graves of Rohingya were discovered in Thailand. The victims had paid smugglers to get on boats but were killed when their relatives could not comply with ransom demands.

But authorities in Bangladesh say that the trade never stopped as smugglers turned to new ways to get people out of the camps by air and land to less appealing destinations such as Pakistan, India and Nepal.

Smugglers arrange fake passports and birth certificates for Rohingya, a stateless minority denied citizenship and other basic rights in Myanmar, authorities say.

The crisis in the camps is dramatically worsening as new waves of refugees claim Myanmar security forces have redoubled efforts to drive remaining Muslims from their homes.

Agence France Presse reports that after a lull the numbers are again swelling with Bangladesh reporting up to 5000 civilians now crossing the border each day.

An estimated 10,000 have reportedly massed at the border, waiting to cross.

Relief agencies warn that if the exodus continues at the same rate there could be one million refugees in the Bangladesh camps by the end of the year, making it the world's largest refugee population.

"We shouldn't let the numbers of this crisis numb us to the fact that everyone of these numbers represents a human life, the majority of whom are children - children who need our urgent help," said Anthony Lake, executive director of the UN children's agency UNICEF.

Mark Lowcock, the UN's under-secretary-general for humanitarian affairs said, the conditions people are living in are horrific.

"It is vital that we do all we can to make sure that the current refugee tragedy does not now become a catastrophe with the outbreak of disease."

The humanitarian crisis in numbers (Source UN, UNICEF):

• More than half a million new refugees have crossed into Cox's Bazar since 25 Augus
  
• The total amount of land now occupied by refugees is 9.6 million square metres. This is equivalent to 889 football fields
  
• Up to 60 per cent of the new arrivals are children, and 30 per cent are children under five years old. Seven per cent are infants under one
  
• Three per cent of the newly arrived refugees are pregnant, and 7 per cent are breastfeeding women
  
• Up to one in five refugee households are headed by women, and 5 per cent by children
  
• Up to 90 per cent of new arrivals have reported eating just one meal a day
  
• One out of five children under the age of five suffers from acute malnutrition
  
• More than 1600 unaccompanied and separated children have been identified and are being provided with support
  
• More than 200,000 newly arrived Rohingya children need access to education
  
• Over half the refugees are living in sites with no access to healthcare
  
• 320,000 refugees need immediate access to clean water and sanitation; and more than 3000 cubic litres of safe water are needed every day
  
• A least 4790 cases of diarrhoea have been reported in the past week. WHO has warned of the potential for an outbreak of cholera
  
• More than half of the gender-based violence cases that have been reported by Rohingya refugees in Cox's Bazaar, Bangladesh, are sexual assault
  
• Approximately 15 million food rations are required every month; 150,000 women and children need support to prevent and treat malnutrition
  
• Since the start of the crisis, the UN and partners have delivered 9 million food rations, provided 190,000 people with emergency shelter kits, and provided 159,000 people with access to healthcare
  
• Over 400 tube wells and 3000 latrines have been constructed
  
• More than 100,000 people have been vaccinated against measles, rubella, and polio