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Volkswagen's boss has warned that a deal with US authorities over its emissions scandal could take longer and cost more than expected.

Speaking at the Geneva Motor Show, Matthias Mueller told the ´óÏó´«Ã½ that VW was still in "constructive dialogue" with regulators and hoped the firm would be "judged fairly".

He said he was "impatient" for answers.

The VW boss also warned that the €6.7bn (£5.2bn) set aside to cover the costs of the scandal might not be enough.

"I've postponed our year-end financial results and the AGM to improve their quality, so that we can be even more confident and take even more care and diligence in establishing the figures.

"Then we'll see if we have to make additional provisions, over and above the 6.7bn [euros]," he added.

Mr Mueller said an internal inquiry could be ready in April.

Last September, US authorities discovered that VW used computer software to massage emissions data during tests, sparking the biggest crisis in VW's history and leading to the departure of Mr Mueller's predecessor.

The US Justice Department is suing VW for breaching environmental laws, and VW has been ordered to fix almost 600,000 diesel vehicles in the country fitted with so-called "defeat devices".

However, about 11 million vehicles globally have been fitted with the devices.

Mr Mueller told the ´óÏó´«Ã½ that it had been a "grave mistake… We have lost a lot of trust with our customers, and we now need to win them back".

He added: "We need to face the allegations of the authorities, and I expect a fair ruling here. And then it's important to look ahead, and to make Volkswagen into an even better company."

But he said he would not pre-empt the internal inquiry underway by lawyers Jones Day into what management knew about the emissions issues before it was exposed.

After publication of the report "we will know the whole story, and the truth will be on the table," Mr Mueller promised.

"It is simply expedient that we carefully consider the whole situation to bring the truth to light. The complexities require time and care," he said.

He insisted, however, that VW had not been "paralysed" by the emissions crisis and that it was looking to "the future with confidence".