The World Health Organization Tuesday urged tighter regulation of privately sold vaccines in China as a scandal involving out-of-date immunisation fuels public outrage in the country's latest safety scare.

The case involves the illegal and improper storage, transport and sale of tens of millions of dollars' worth of vaccines -- many of them expired -- reports say.

"The vaccines that are in the private sector need to be managed, stored, handled, distributed and used in accordance with recognised standards," Lance Rodewald, a WHO expert on immunisation, told a briefing.

"This is a very serious situation, it's being taken seriously. We take it seriously. We want to see the root causes identified so that remedies can be provided."

Standard vaccines such as those for polio, hepatitis B and measles are mandatory for all children in China and supplied by the state, while parents can opt to buy additional immunisations privately such as those for meningitis, influenza or rotavirus.

China's public vaccination system is fundamentally sound, Rodewald said, adding that the expired private-sector vaccines did not pose a threat to children who received them.

"Parents should be comfortable knowing that their child will not have a toxic reaction," he said, although some children "may need to be revaccinated".

Authorities have already arrested more than 130 suspects in the investigation into the sales.