LONDON -- An estimated 40 percent of the dead or missing from the Myanmar cyclone are believed to be children, international charity Save the Children said Wednesday.

The agency said its workers on the ground believe that the casualty toll could be "much higher" than the official toll of 22,000 dead and one million homeless, comparing it to the 2004 Asian tsunami.

"It is a race against time and now our priority has to be those who are left -- we urgently need help to be able to reach the surviving children and families," said Andrew Kirkwood, the charity's head in Myanmar.

He noted that the storm surge in many parts of the country's low-lying delta region was reportedly as high as 25 feet (7.5 meters) in places, "and as the delta is a very flat area we expect that many, many people drowned."

"About 40 percent of the people living in the delta are children under 18, so we would expect that 40 percent of the dead and missing are children," he said, cited in a statement.

A vast swathe of Myanmar's low-lying delta region was inundated by the storm which hit Saturday, sweeping away entire towns, and triggering fears that disease could push the death toll still higher.

Pledges of cash, supplies and assistance are pouring in from around the world, but little is reaching the reclusive country, and experts are warning of a catastrophe if they are not allowed in to direct the relief effort.

"We know that some areas are still completely under salt water. Some people have no drinking water or food. Unless assistance gets into those kinds of areas very soon, the death toll will keep rising," said Kirkwood.