Agents went undercover to gain access to the chat room

Twenty-seven people - from Australia, Britain, Canada and the US - have been charged with participating in two online chat rooms.

One of the children used by the ring was less than 18 months old, US Attorney General Alberto Gonzales said.

It was "the worst imaginable form of child pornography", he said.

'Insidious network'

Federal agents went undercover to infiltrate the chat room, in a 10-month investigation led by the US Immigration and Customs Enforcement.

They "revealed an insidious network that engaged in worldwide trafficking in child pornography, including live molestations of children transmitted over the Internet," Mr Gonzales said in a statement.

They identified seven children, including the infant, who were molested live on a streamed video via the internet chat room.

Four of the charged are accused of molesting the children.

Thirteen of the suspects, from nine US states, faces charges that include possession, receipt, distribution and the manufacture of child pornography.

Of the other 14, nine have been charged in Canada, three in Australia and two in Britain. One person is still at large.