The plane has been grounded since March following two crashes. (File)

The United States aviation regulator is under no time pressure to give permission for the troubled Boeing 737 MAX to return to the skies, its chief said Wednesday.

The plane has been grounded since March following two crashes, of Lion Air and Ethiopian Airlines flights, that left a total of 346 people dead.

Boeing has said it is working hard to introduce the necessary changes to the aircraft as it sets goals for its return to service.

However, US Federal Aviation Administration (FAA) administrator Steve Dickson said that a January date set by Boeing "remains to be seen".

"We are not following a timeline" on the 737 MAX's return to the skies, he told a press conference at the Dubai Airshow, adding that the FAA was ensuring the re-certification process was "very methodical, very diligent".

"We are completely focused on the 737 MAX situation right now," he said.

"I am absolutely committed to honouring the memory of those who lost their lives in the accidents... by working relentlessly to ensure the highest standards of safety in the global aviation system."

Boeing set a timeline for a possible re-certification of the plane by the end of the year and a tentative return to service in January, but it has said that the deadlines are in the hands of the regulators.

Mr Dickson said the FAA has involved nine global regulators, including that of the United Arab Emirates, which is hosting the Dubai Airshow, in the process of the 737 MAX re-certification.

The schedule for lifting the grounding has slipped several times, as regulators have raised fresh questions about upgrades and the company and the FAA have faced scrutiny over the MAX.

Last month, Boeing Chief Executive Dennis Muilenburg endured two days of bruising congressional hearings, during which lawmakers accused the company of cutting corners on safety to rush out the MAX to compete with an Airbus model.

The probes have centred on a Boeing flight handling system, the MCAS, that has been implicated in both crashes.