The impact of Cyclone Nargis in Burma will have a longer lasting effect than the tsunami which devastated Asia in 2004, says World Vision Australian CEO Tim Costello.
Speaking from Rangoon, Mr Costello said the effects of the cyclone were "huge".
"Its impact on this country we can definitively say will be more lasting than the tsunami was," Mr Costello said.
"Its crushing impact fearfully means that many of those unaccounted for are probably people who have died."
And while aid was getting through, it was still "not commensurate to the need", he said.
"The impression that no aid is getting through is wrong. We are getting aid to some of the most far-flung areas.
"But we know there are still people waiting. We know there are threats of epidemics."
Mr Costello said he had seen World Vision staff in tears, guilty they can't get to everyone.
"The waterborne diseases are just potentially rampant. So the next 24 to 48 hours are absolutely critical," Mr Costello said.
"What we do know is that every hour of delay in terms of getting water and sanitation experts in ... is critical to the outbreak of cholera and other diseases."
Mr Costello said he was heartened by reports that Burma's ambassador to the UN, Kyaw Tint Swe, may "throw the door open" to international aid.
"That would be the most significant help so we can get in and ... do this job."
However, Mr Costello said aid agencies had also been getting mixed signals on when more aid workers would be allowed in the country.
"There is sometimes encouragement on one level ... then the door that seems ajar slams shut again," Mr Costello said.
"The stop-start process is just very frustrating."
Mr Costello said it was important the Burmese government was assured aid agencies were only in the country for humanitarian reasons.
Around 300 international aid workers, including four World Vision workers from Australia, were waiting for visas to enter Burma.
"They (the aid workers) are waiting in Bangkok. There are cargo planes waiting there in Singapore. Ours is there in Dubai with 40 tonnes of supplies which is waiting clearance," Mr Costello said.
"Literally, it is all cued up waiting desperately for the Myanmar (Burma) government to push the button."
Mr Costello said there would be less than 20 international aid workers currently in Burma.
"We know from the tsunami operation that with the right tools - the helicopters, the planes, the permits - we can get to these people, stop epidemics (and) we can save lives. We know this is possible."
The 2004 Indian Ocean tsunami killed 230,000 people in a dozen nations.
