The US Central Intelligence Agency (CIA) has a long history of involvement in African affairs, so Sunday's reports that the 1962 arrest of Nelson Mandela came following a CIA tip-off don't come as a huge surprise.

Most incidents came during the Cold War, when the US and the Soviet Union battled for influence across the continent.

CIA covert operations are by their very nature hard to prove definitively. But research into the agency's work, as well as revelations by former CIA employees, has thrown up several cases where the agency tried to influence events.

Here are four examples:

1) 1961 - Patrice Lumumba's assassination in Congo

Patrice Lumumba became the first prime minister of the newly-independent Congo in 1960, but he lasted just a few months in the job before he was overthrown and assassinated in January 1961.

In 2002, former colonial power Belgium admitted responsibility for its role in the killing, however, the US has never explained its role despite long-held suspicions.

US President Dwight D Eisenhower, concerned about communism, was worried about Congo following a similar path to Cuba.

According to a source quoted in Death in the Congo, a book about the assassination, President Eisenhower gave "an order for the assassination of Lumumba. There was no discussion; the [National Security Council] meeting simply moved on".

However, a CIA plan to lace Lumumba's toothpaste with poison was never carried out, Lawrence Devlin, who was a station chief in Congo at the time, told the BBC in 2000.

A survey of declassified US government documents from the era notes that the CIA "initially focussed on removing Lumumba, not only through assassination if necessary but also with an array of non-lethal undertakings".

While there is no doubt the CIA wanted him dead, the survey does not indicate direct US involvement in his eventual killing.

2) 1965 - Overthrow of Kwame Nkrumah in Ghana

Ghana's first President Kwame Nkrumah was overthrown in a military coup in 1966 while he was out of the country.

He later suspected that the US had a role in his downfall and in a 1978 book, former CIA intelligence officer John Stockwell backed this theory up.

In In Search of Enemies he writes that an official sanction for the coup does not appear in CIA documents, but he writes "the Accra station was nevertheless encouraged by headquarters to maintain contact with dissidents.

"It was given a generous budget, and maintained intimate contact with the plotters as a coup was hatched."

He says that the CIA in Ghana got more involved and its operatives were given "unofficial credit for the eventual coup".

A declassified US government document does show awareness of a plot to overthrow the president, but does not indicate any official backing.

Another declassified document written after the coup describes Nkrumah's fall as a "fortuitous windfall. Nkrumah was doing more to undermine our interests than any other black African".

3) 1970s - Opposition to the MPLA in Angola

In Angola three competing groups fought for control after independence from Portugal in 1975, with the MPLA under Agostinho Neto taking over the capital Luanda.

Mr Stockwell, chief of CIA's covert operations in Angola in 1975, writes that Washington decided to oppose the MPLA, as it was seen as closer to the Soviet Union, and support the FNLA and Unita instead, even though all three had help from communist countries.

The CIA then helped secretly import weapons, including 30,000 rifles, through Kinshasa in neighbouring Zaire, now known as the Democratic Republic of Congo, Mr Stockwell says in a video documentary.

He adds that CIA officers also trained fighters for armed combat.

A declassified US government document detailing a discussion between the head of the CIA, the secretary of state and others indicates the support the CIA gave to the forces fighting the MPLA.

The US continued to support Unita through much of the civil war as Cuba was backing the MPLA.

4) 1982 - Supporting Hissene Habre in Chad

Hissene Habre failed in his attempt to take power by force in Chad in 1980.

But his efforts led President Goukouni Oueddei to call on help from the Libyan leader Muammar Gaddafi, whose soldiers successfully beat back Habre's challenge and forced him into exile.

A proposed alliance between Libya and Chad began to unsettle the US especially as Gaddafi began to be seen as a supporter of anti-US activities.

In Foreign Policy magazine Michael Bronner writes that the CIA director, with the secretary of state, "coalesced around the idea of launching a covert war in partnership with Habre".

It is alleged that the US then backed Habre's overthrow of the president in 1982 and then supported him throughout his brutal rule.