A Pakistani-based branch of Al-Qaeda was hatching a plot to kill the head of US defense group Lockheed Martin, self-confessed terrorist David Coleman Headley testified in a US court Tuesday.

The planned assassination was in retaliation for the Lockheed Martin-made drones, Headley testified during the Chicago trial of his childhood friend, Tahawwur Hussain Rana.

Headley pleaded guilty to 12 terrorism charges related to the deadly 2008 Mumbai attacks and other unrealized plots in the wake of his 2009 arrest in Chicago.

He is testifying against alleged co-conspirator Rana in exchange for avoiding the death penalty and extradition to India, Pakistan or Denmark.

Headley testified that he secretly used Rana's office computer for research on the plot to assassinate the Lockheed Martin executive but dismissed his brief online search there as insignificant.

"My research is more in-depth than Googling someone a couple of times," he testified during cross-examination by Rana's defense attorney.

Headley said he was working on the plot with Ilyas Kashmiri, the commander of the Pakistani-based terrorist organization Harakat-ul-Jihad al-Islami (HUJI), and a senior member of Al-Qaeda.

Headley pleaded guilty to working with Kashmiri on a plot to attack the Danish newspaper Jyllen Posten, which published controversial cartoons of the prophet Mohammed, after Lashkar-e-Taiba (LeT) got distracted with the Mumbai plot.

A spokeswoman for Lockheed Martin declined to comment on whether the group had been informed of the alleged plot or taken any additional measures to protect Robert Stevens, who has been chief executive officer since 2004.

"Our number one concern is the safety and security of every Lockheed Martin employee," Jennifer Whitlow told AFP.

"We base decisions about employee and facility security measures on our ongoing assessment of the security environment but as a matter of policy, we do not discuss specific threats against our company or our employees."

Rana is accused of providing Headley with a cover and acting as a messenger, with prosecutors alleging he played a behind-the-scenes logistical role in both the Mumbai attacks and another abortive plan to strike Copenhagen.

Rana, a Canadian-Pakistani and Chicago businessman, has denied all charges, and his defense attorneys argue that he was duped by his friend, whom he had met in military school.