Volkswagen's former CEO was alerted to US diesel emissions tests problems in 2014, but the issue "did not initially receive particular attention at the management levels," Reuters reported the carmaker as saying.

A memo was sent to former chief Martin Winterkorn in May 2014 about a study by US researchers that raised questions about whether Volkswagen diesel cars were emitting more smog-creating pollutants in real-world driving than in standard government tests, the company said in a statement that summarised its response to a shareholder lawsuit in Germany.

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That memo was included in Winterkorn's "extensive weekend mail", VW said. It did not specify who sent the memo, and did not say whether Winterkorn actually read it.

"Whether and to which extent Mr. Winterkorn took notice of this memo at that time is not documented," VW said, according to Reuters.

In November 2014, Winterkorn received another memo that referred to a cost framework for the diesel issue in North America, then estimated at EUR20m.

The former CEO also attended a July 2015 meeting at which diesel emissions issues were discussed, but it is not clear whether he knew of the cheating at the time, the news agency said.

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"Volkswagen expressly regrets that, looking back, the situation is different," the company statement said.

VW must tell a US judge whether it has an acceptable fix for almost 600,000 diesel cars by 24 March, Reuters noted.

A report VW commissioned is due next month from US law firm Jones Day which is investigating those responsible for the biggest corporate scandal in its history. The automaker is also facing lawsuits claiming it was too slow to inform investors about its rigging of emissions tests.