The men were shot dead in a car in Karachi last November in what was described at the time as a highly organised operation.

Killings may have been a response to conviction of Mir Aimal Kansi

The executives of a Texas-based oil company were killed after a Pakistani citizen was convicted last year in America of murdering two CIA agents.

Pakistani police said they believed the killings may have been a direct response to the conviction of Mir Aimal Kansi who shot the two employees outside the CIA's headquarters in Virginia in 1993.

The advertisement, placed in Pakistan's main newspapers, says America might be prepared to relocate the individual and family members of those who provide information.

It is not the first time America has offered such a reward. It did so following the murder of two US diplomats in Karachi in March 1995.

Although that particular crime remains unsolved, American officials say the offers of rewards in Pakistan have worked in some cases and hope that the prospect of obtaining up to $2m will tempt someone to provide evidence about the deaths of the Union Texas employees.