Cuban intelligence services directly advise Venezuelan president Hugo Chavez in what a US diplomat called the "Axis of Mischief," according to a State Department cable released by the WikiLeaks website.
Other releases by the group revealed US anxiety at Mr Chavez's "coziness" with Iran, and concerns by Venezuelan Jews over what they see as government prejudice against them.
Worries over Cuba's role in Venezuela were shown in a 2006 diplomatic message. "Cuban intelligence has much to offer to Venezuela's anti-US intelligence services," said the cable posted on wikileaks.org yesterday.
During 12 years in office, Mr Chavez has forged close ties with Cuba's Castro brothers, subsidising the communist island's economy with cheap oil in return for thousands of doctors and advisers who operate in Venezuela.
A former paratrooper, Mr Chavez has incorporated Cuban-style militias in the armed forces and experts on Venezuela have long said Cuban intelligence services train the president's security detail.
The leaked document implied the president trusts Cuban information almost more than his own intelligence services. "Cuban intelligence officers have direct access to Chavez and frequently provide him with intelligence reporting unvetted by Venezuelan officers," the report said.
"Sensitive reports indicate Cuban and Venezuelan intelligence ties are so advanced that the two countries' agencies appear to be competing with each other for the BRV's (Bolivarian Republic of Venezuela's) attention."
The cable was part of a cache of more than 250,000 State Department documents that WikiLeaks released either to media outlets or on its website this week.
It did not reveal the sources behind the "sensitive reports." The document was classified by diplomat Robert Downes, the US embassy's then political counsellor in Caracas.
It was titled "Cuba/Venezuela Axis of Mischief: The view from Caracas," in an apparent reference to former US President George W. Bush's "axis of evil" - a term for three countries he accused of supporting terrorism.
Another in a clutch of Caracas embassy cables released by WikiLeaks showed the vulnerability felt by Venezuela's Jewish community given Mr Chavez's political opposition to Israel, with which he broke relations in 2009.
Mr Chavez denies being anti-Semitic, but his fierce words against Israel have been taken by some supporters as a green light for actions like daubing walls with anti-Jewish slogans.
"They believe he has merged his anti-Zionist views with anti-Semitic ones," the 2009 cable said, describing opinions of local Jewish leaders. "The horizon is dark," it quoted one unnamed leader as saying of religious freedom in Venezuela.
Another cable from 2006 dealt with Mr Chavez's blossoming friendship and "bilateral coziness" with Iranian president Mahmoud Ahmadinejad. "A shared hatred for the USG (US government) is the driving force" said the cable, describing "Chavez's ill-defined, left-wing, anti-American ideology."
The cable said Venezuela's backing for a nation accused of supporting terrorism and talking of eliminating Israel was a matter of "grave concern."
But rumours of Venezuelan cooperation with a suspected Iranian nuclear arms plan "appear baseless" and "little more than conspiracy-mongering by Chavez adversaries," it said.
Other cables, from 2009, said Venezuela's frequent talk of developing a nuclear energy program was largely scoffed at by local physicists due to the lack of domestic expertise and the enormous financing needed.
Mr Chavez has since said Russia will provide it with a nuclear power plant, but the scepticism remains.
Reuters