The US Justice Department said Monday that it was "extremely disappointed" in a British judge's decision not to extradite WikiLeaks founder Julian Assange to the United States to face trial for publishing US secrets.

"While we are extremely disappointed in the court's ultimate decision, we are gratified that the United States prevailed on every point of law raised," the department said.

It pointed to the judge's rejection of Assange's claims that he had been exercising free speech rights when he dumped thousands of leaked classified US military and diplomatic files onto the internet in 2010, and that the US was pursuing a political vendetta because of it.

Instead, the judge ruled that Assange's mental health is fragile and that he would be at significant risk of suicide in an American prison. 

The judge said it was not evident that the United States, keeping him jailed while awaiting trial, would be able to ensure his safety in prisons known for "harsh conditions."

Assange was indicted for violating the US espionage act in leaking the US files, and for hacking, based on the alleged assistance he provided former military intelligence officer Chelsea Manning in obtaining the documents from secure military computer systems.

But the US case raised free speech issues, with Assange and defenders maintaining that WikiLeaks enjoys the rights of any other media to publish secret materials.

If found guilty on the US charges, he could face spending the rest of his life in prison.

But Assange had sought a pardon from President Donald Trump, whose 2016 election campaign benefited from WikiLeaks' release of materials that damaged Trump's opponent, Hillary Clinton.

The Justice Department said it will appeal the decision and continue to seek Assange's extradition, while Mexican President Andres Manuel Lopez Obrador said his county would offer him asylum.