The Obama administration has denounced the leaking of mountains of classified material by WikiLeaks, calling it a serious crime and an attack on the whole world.

Secretary of state Hillary Clinton says the US "deeply regrets" Monday's embarrassing release of more than 250,000 state department cables.

But she says "there is nothing brave about sabotaging the peaceful relations between nations".

"I will not comment on or confirm what are alleged to be stolen state department cables," Ms Clinton said.

"But I can say that the United States deeply regrets the disclosure of any information that was intended to be confidential, including private discussions between counterparts or our diplomats' personal assessments and observations."

Ms Clinton has vowed to prevent future leaks and find those responsible for the current crisis.

"This disclosure is not just an attack on America's foreign policy interests. It is an attack on the international community," she said.

"I would also add that to the American people and to our friends and partners, I want you to know that we are taking aggressive steps to hold responsible those who stole this information."

Ms Clinton's comments came as the White House ordered tighter security to prevent future leaks.

Press secretary Robert Gibbs said president Barack Obama was decidedly "not pleased" by the WikiLeaks release, adding those responsible were "criminals" who had committed a serious offence.

"This is a serious violation of the law, a serious threat to individuals that both carry out and assist in our foreign policy," Mr Gibbs said.

Earlier, the US Justice Department said authorities were conducting a criminal investigation into the leak of the classified documents, which WikiLeaks provided to five media groups that published reports on them.

Security crackdown

Attorney-general Eric Holder has promised there will be prosecutions if US law has been broken.

"Let me be very clear, this is not sabre rattling. This is, as I said, an active ongoing investigation to the extent that we can find anybody who was involved in the breaking of American law and who has put at risk the assets and the people that I have described, they will be held responsible. They will be held accountable," he said.

And in a direct nod to WikiLeaks founder Julian Assange, Mr Holder says everyone is a target, whether they are an American citizen or not.

The incoming chairman of the House Homeland Security Committee, Republican Peter King, wants even tougher action.

"I'm also calling on secretary of state Hillary Clinton to declare WikiLeaks a foreign terrorist organisation. By doing that we will be able to seize their funds and go after anyone who provides them with any help or contributions or assistance whatsoever," he said.

"To me they are a clear and present enemy of the United States of America."

Australia cables

Meanwhile, Defence Minister Stephen Smith has been briefed by US ambassador Jeffrey Bleich on the contents of about 1,500 cables that mention Australia.

One cable already released about the situation in Zimbabwe describes Australia as rock solid but largely uninfluential.

Mr Smith has downplayed any potential embarrassment from the contents of the cable.

"You can't take a pin prick from an individual cable to get a general assessment," he told Radio National.

"When I have my conversations with my counterparts, whether it's secretary of defence Robert Gates or secretary of state Clinton, Australia is held in very high regard for the role we play internationally."

Mr Smith would not comment on claims made in another cable about concerns that Australians who had gone missing in the Middle East had ended up on US terrorist watch lists.

A whole of government taskforce is now combing through all the documents to assess any national security implications for Australia.

Candid cables

Monday's release of documents obtained by WikiLeaks exposed the inner workings of US diplomacy in recent years, including candid assessments of world leaders and disclosures on issues such as Iran's nuclear and missile programs.

Among the revelations was that Saudi King Abdullah repeatedly urged the United States to attack Iran's nuclear program. The documents cited him as saying: "cut off the head of the snake," according to the Guardian newspaper of Britain.

The New York Times also reported impolitic comments about foreign leaders, including a description of Russian president Dmitry Medvedev, Russia's head of state, as playing "Robin to (prime minister Vladimir) Putin's Batman."

The White House, which harshly condemned the release and said the disclosures may endanger US informants abroad, has ordered government agencies to tighten procedures for handling classified information.

The new procedures will ensure "that users do not have broader access than is necessary to do their jobs effectively," and will put restrictions on the handling of classified material, the White House Office of Management and Budget said.

Before Monday, WikiLeaks had made public nearly 500,000 classified US files on the wars in Iraq and Afghanistan.

The US investigation into the source of those leaks has focused on Bradley Manning, a former US army intelligence analyst in Iraq.

Manning is under arrest, charged with leaking a classified video showing a 2007 helicopter attack that killed a dozen people in Iraq, including two Reuters journalists.

- ABC/wires