A Democratic senator in the US has insisted former CEOs of carmaker General Motors should be questioned over the company's slow response to the ignition switch defect, which resulted in at least 13 deaths and millions of car recalls globally.

"Of course we have to go back to the prior CEOs," Reuters quoted Senator Richard Blumenthal of Connecticut, a member of the Senate Commerce Committee investigating the issue, as saying.

He said there are "a whole set of questions on why there were so many delays in taking action to remedy this situation".

Nevertheless, he noted that it will be up to the Senate Commerce subcommittee Chairwoman Claire McCaskill to schedule the next hearings and decide who will testify.

In a Congressional hearing earlier in April, GM CEO Mary Barra apologised to the relatives of the 13 people killed in car accidents due to faulty ignition switches, calling her company's slow response to the deaths "unacceptable".

Barra, who became GM CEO in January, said she has no idea why it took the company so long to disclose the ignition switch defect, and noted that an internal investigation into the matter was going on.

GM was accused of taking no steps related to the issue until February, despite learning about it in 2011. Recently, GM, the second-largest automaker in the world, recalled 6.3 million vehicles, including the 2.6 million vehicles in connection with the faulty ignition switches.

US Congress is investigating why GM and the industry regulators missed or ignored a number of red flags that faulty ignition switches could unexpectedly turn off engines during operation, leaving airbags, power steering and power brakes inoperable.

Earlier, the US National Highway Traffic Safety Administration (NHTSA) fined GM for having failed to respond to requests for information about the defect.

Separately, GM said it is restructuring its engineering operations in order to improve quality and safety of its vehicles, and that its global vehicle engineering chief, John Calabrese, is retiring.

The company had placed two engineers on paid leave in connection with its internal investigation into the ignition switch defect.