Three of Australia's biggest Internet service providers agreed Friday to voluntarily block child pornography online before the government introduces its mandatory filter, Communications Minister Stephen Conroy said.

Telstra, Optus and Primus will block a list of URLs compiled by the government's Communications and Media Authority.

But the companies would not confirm their support for the proposed filter, which would make Australia one of the strictest Internet regulators among the world's democracies. The filter has been criticized by Google and Yahoo, and even the U.S. State Department expressed concern about the restrictions.

Conroy also announced a review would be conducted into the guidelines for the banned content under that filter. The restricted content includes child abuse material, bestiality, rape and other extreme violence and terrorist acts.

"We support the review that was announced today, we support and are willing to voluntarily commit to the blocking of the ACMA list of child pornography sites and we'll continue to work constructively with the government as it undertakes this review," Telstra public policy and communications director David Quilty told reporters in Melbourne.

Optus government and corporate affairs director Maha Krishnapillai said the company had agreed to block child pornography where it could.

"We'll have to wait and see what the review comes out with, but we've said all the way through this is about blocking the worst of the worst," he said.

Quilty said it could take several months to begin blocking the child pornography sites.

The Internet filter proposal needs the support of Parliament to become law later this year.

Some critics of the proposed filter have said it puts the nation in the same censorship league as China, and Internet giants Google and Yahoo have called it heavy-handed.

The U.S. State Department said in April that open Internet access encouraged economic prosperity and the free flow of information.