China National Chemical Corp (ChemChina) offered A$2.96 billion (US$2.7 billion) for Australian chemical maker Nufarm Ltd and invited two US private equity firms to join the bid, a source said.

ChemChina submitted a letter to Nufarm on Wednesday with an indicative bid of A$17.25 per share, said the source, declining to be identified.

The company asked Blackstone Group LP, manager of the world's biggest buyout fund, and Fox Paine & Co to take minority stakes in Nufarm, the source said.