They include reports of some Arab leaders - including Saudi Arabia's King Abdullah - urging the US to attack Iran and end its nuclear weapons programme.
Other concerns include the security of Pakistani nuclear material that could be used to make an atomic weapon.
The widespread use of computer hacking by China's government is also reported.
The US government condemned the release of the documents, which number in the hundreds of thousands, saying they put the lives of diplomats and others at risk.
The founder of Wikileaks, Julian Assange, countered by saying the US authorities were afraid of being held to account.
The leaked US embassy cables, published at length in newspapers including the New York Times and the UK's Guardian, also reportedly include accounts of:
- Iran attempting to adapt North Korean rockets for use as long-range missiles
- Corruption within the Afghan government, with concerns heightened when a senior official was found to be carrying more than $50m in cash on a foreign trip
- Bargaining to empty the Guantanamo Bay prison camp - including Slovenian diplomats being told to take in a freed prisoner if they wanted to secure a meeting with President Barack Obama
- Germany being warned in 2007 not to enforce arrest warrants for US Central Intelligence Agency officers involved in an operation in which an innocent German citizen with the same name as a suspected militant was abducted and held in Afghanistan
- US officials being instructed to spy on the UN's leadership by Secretary of State Hillary Clinton
- The very close relationship between Russian PM Vladimir Putin and his Italian counterpart Silvio Berlusconi
- Alleged links between the Russian government and organised crime
- Yemen's president talking to then US Mid-East commander General David Petraeus about attacks on Yemeni al-Qaeda bases and saying: "We'll continue saying the bombs are ours, not yours"
- Criticism of UK politicians including Prime Minister David Cameron
- Faltering US attempts to prevent Syria from supplying arms to Hezbollah in Lebanon
In a statement, the White House said: "Such disclosures put at risk our diplomats, intelligence professionals, and people around the world who come to the United States for assistance in promoting democracy and open government.
"President Obama supports responsible, accountable, and open government at home and around the world, but this reckless and dangerous action runs counter to that goal."
Earlier, Wikileaks said it had come under attack from a computer-hacking operation.
"We are currently under a mass distributed denial of service attack," it reported on its Twitter feed.
No-one has been charged with passing the diplomatic files to the website but suspicion has fallen on US Army private Bradley Manning, an intelligence analyst arrested in Iraq in June and charged over an earlier leak of classified US documents to Mr Assange's organisation.
Wikileaks argues that the site's previous releases shed light on the wars in Afghanistan and Iraq.