HONG KONG (Reuters) - Some children who have fallen ill in China after being fed milk formula that had been contaminated with melamine have developed “crystals” in their kidneys, a WHO food safety expert said on Tuesday.

“Our understanding is that these are not normal kidney stones because they are not being detected via all of the tools that one uses to detect kidney stones, so some are not showing up on x-rays,” said Anthony Hazzard, regional advisor for food safety at the World Health Organization.

“We believe at this stage that it’s really the complex of melamine and cyanuric acid forming what you call crystals ... they can form in the small tubules (in the kidneys) and they get bigger and can block the tubules,” he said in a telephone interview from Manila.

Cyanuric acid is chemically similar to melamine - a plastic-making industrial compound that was added to milk powder to cheat quality tests.

Tens of thousands of Chinese children have fallen ill with kidney stones and other kidney problems after consuming the contaminated milk, and at least four children have died.

It is not clear how cyanuric acid found its way into the children. Experts say it could have been added along with melamine into milk, also to cheat quality tests, or it may have been formed when the body tried processing the melamine, which is indigestible.

Experts said earlier that crystals may be even more troublesome than stones. They can potentially impair kidney function especially when large numbers of them suddenly form at the same time, blocking tubules in the kidneys.

Hazzard said the children were mainly treated in three ways.

“Depending on how severe the situation, they can either pass these crystals in urine or they need hospitalization and treatment with fluids that will allow these crystals or stones to better pass, or in some cases, they may need surgery,” he said.

This latest China food scandal has sparked tests for melamine in a variety of Chinese-made products from milk and chocolate bars to yoghurt exported around the world, including products in South Korea, leading to items being pulled from shop shelves.