Volkswagen's drawn-out diesel emissions scandal has prompted another apology from the German car-maker after media reports that it had been complicit in allowing monkeys to inhale car fumes to test the toxicity of diesel emissions.

"We are convinced, that the scientific methods chosen at the time were wrong," said the company in a statement on Saturday. "It would have been better to forgo such a test from the very beginning."

It also denounced all forms of animal abuse. "We ask forgiveness for this bad behaviour and for the poor judgment of some individuals."

The New York Times broke the story this week about an Albuquerque research facility where 10 monkeys were locked in an airtight container, watching cartoons to distract them as they breathed in fumes from a Volkswagen Beetle.

Tying this scandal to the larger one, the Beetle in question had been rigged to emit fewer fumes during testing scenarios, an effort to spoof tests. The 2015 revelation of the practice has been the source of international litigation and market problems for the company ever since.

The test was conducted by the Lovelace Respiratory Research Institute in the US state of New Mexico. German car-makers Daimler and BMW also supported the project, though VW had the lead in organisation with the help of a European research institute.

The German state of Lower Saxony, which is also a major owner of Volkswagen, also reacted with shock on Saturday and demanded full disclosure about the 2014 tests. It also demanded assurances that such tests never happen again.

"Letting 10 monkeys breathe in car emissions for hours to prove that there has been a reduction in the amount of poisonous emissions is horrid and absurd," said state Premier Stephan Weil.