The Solar Impulse 2 experimental plane was on its way to the south-central US state of Oklahoma late Thursday, as it pressed ahead with its record-breaking quest to circle the globe without consuming a drop of fuel.

The experimental solar-powered aircraft, piloted by Swiss adventurer Bertrand Piccard, took off at 3:00 am (1000 GMT), for the latest stage of its around-the-world flight aimed at promoting clean energy technologies.

The flight from Phoenix, Arizona to Tulsa was expected to take about 18 hours.

As of Thursday evening, the plane was about 85 percent of the way there, flying at 18,000 feet (5,500 meters), north of Oklahoma City, Oklahoma.

"We will soon start the descent after the Air Traffic Controler confirms us," a post from the plane's Twitter account said.

A photo tweeted earlier from the aircraft showed a view of its tail as Solar Impulse flew in a sunny sky, above small puffs of white that cast shadows on the ground.

- Objective: New York, ASAP -

The plane will make one or two more stops before landing in New York, in the latest leg of a journey that kicked off in Abu Dhabi on March 9, 2015.

"The objective is to reach New York as soon as possible!" the Solar Impulse 2 team said in a statement Wednesday, although it is not clear when the plane might reach there.

Thanks to an inflatable mobile hangar, which can be packed up quickly and transported, Solar Impulse 2 can be sheltered at a variety of locations.

The aircraft was grounded in July last year when its batteries suffered problems halfway through its 21,700-mile (35,000-kilometer) circumnavigation.

The crew took several months to repair the damage from high tropical temperatures during a 4,000-mile (6,437-kilometer) flight between Nagoya, Japan and Hawaii.

The plane was flown on that stage by Piccard's teammate Andre Borschberg, whose 118-hour journey smashed the previous record of 76 hours and 45 minutes set by US adventurer Steve Fossett in 2006.

He took catnaps of only 20 minutes at a time to maintain control of the pioneering plane during the flight from Japan, in what his team described as "difficult" conditions.

The Solar Impulse 2, which weighs roughly the same as a family car but has wings wider than those of a Boeing 747, contains 17,000 solar cells that power the aircraft's propellers and charge batteries.

At night, the plane runs on stored energy.

The typical flight speed is around 30 miles (48 kilometers) an hour, which can increase to double that when exposed to full sunlight.

- 'Piece of cake' -

"After your three-day flight over the Pacific, this 17-hour one to Tulsa should be a piece of cake," Borschberg said in a tweet to Piccard ahead of takeoff, referring to an earlier leg from Hawaii to San Francisco.

After crossing the United States, the pilots are set to make a trans-Atlantic flight to Europe, from where they plan to make their way back to their point of departure in Abu Dhabi.

Piccard, a doctor by training, completed the first non-stop round-the-world balloon flight in 1999.

His teammate Borschberg is no stranger to adventure -- 15 years ago he narrowly escaped an avalanche, and in 2013 he survived a helicopter crash with just minor injuries.