The Greens have called on the Federal Government to reveal whether it knew about secret United States charges against WikiLeaks founder Julian Assange.

An email from staff at private US intelligence agency Stratfor, released by WikiLeaks, refers to an indictment on Assange.

"We have a sealed indictment on Assange," said the short email from Stratfor's vice-president of intelligence Fred Burton to analysts at the security firm.

The information comes with the request to protect the information and not to publish.

Greens Senator Scott Ludlam wants Prime Minister Julia Gillard to say whether the Government will defend the 40-year old Australian against possible extradition to the US.

"The Australian Government needs to take a very straight line on this with the Obama administration that we will not permit and we will not tolerate his transfer to the United States to face charges that could potentially land him in prison or in a hole like Guantanamo Bay, as David Hicks did, potentially for decades," he said.

"What we need to know is whether the Australian Government was tipped off, or whether the Prime Minister read about this in The Sydney Morning Herald this morning."

But the Federal Government says it is not aware of any US charges against the WikiLeaks founder.

The Leader of the Government in the Senate Chris Evans says Mr Assange's case is being closely monitored and the claims come as a surprise to him.

"All of that is news to me this morning as well, but I can tell you that the Australian government is not aware of any charges by the US government against Mr Assange," he said.

"Our embassy in Washington continues to closely monitor developments."

Assange is currently under house arrest in England, waiting for a British Supreme Court decision on his appeal against extradition to Sweden on sexual assault allegations.

He strongly denies the claims, saying they are politically motivated and linked to the activities of WikiLeaks.

Assange fears being sent to Stockholm would open the way for his extradition to the US to face charges of spying linked to the leaking of classified military documents by US soldier Bradley Manning.

Manning was formally charged last week for allegedly turning over a trove of classified US documents to WikiLeaks in one of the most serious intelligence breaches in US history.

ABC/AFP