SpaceX's Starship, the most powerful rocket ever built, exploded on Thursday during the first test flight of the spacecraft designed to send astronauts to the Moon, Mars and beyond.

The gigantic rocket successfully blasted off at 9:33pm Hong Kong time from Starbase, the private SpaceX spaceport in Boca Chica, Texas.

The Starship capsule had been scheduled to separate from the first-stage rocket booster three minutes into the flight but separation failed to occur and the rocket blew up.

"We cleared the tower which was our only hope," said Kate Tice, a SpaceX quality systems engineer.

"With a test like this, success comes from what we learn, and today's test will help us improve Starship's reliability as SpaceX seeks to make life multi-planetary," SpaceX tweeted.

Elon Musk’s company was aiming to send the nearly 120-metre Starship rocket on a round-the-world trip from the southern tip of Texas, near the Mexican border.

It carried no people or satellites; both the booster and spacecraft on top were to be ditched into the sea.

Throngs of spectators watched from South Padre Island, several miles away from the Boca Chica Beach launch site. (AFP/AP)