Glass floor cracks on Willis Tower's observation deck 412 metres in the air

The glass floor of an observation deck located on the side of a tower 412 metres in the air has cracked, terrifying tourists.

One of the Skydeck ledges on Chicago's Willis Tower cracked on Tuesday (local time), apparently while tourists were standing on it, CBS News reports.

Video shot from the skyscraper's 103rd floor shows the Skydeck's top layer of glass splintered, but the tower's management says there was no risk to tourists' safety.

It said only the top protective layer of glass was broken and the glass underneath remains stable. Chicago city engineers are also unconcerned, telling CBS News they'll leave it to the company which owns the building to sort it out.

Witnesses aren't happy though, especially people who saw the glass break as visitors were standing on it.

"There was a woman with two kids and they looked really pale and scared because the floor just cracked," witness Jesus Pintado told CBS News.

"I'm scared of heights in general so when I saw that happen, I was like, 'Nope, not going on,'" said witness Karly Pintado.

The glass has now been replaced.

New Zealand tourists had a similar experience recently after a panel in the Sky Tower's observation glass cracked.

"Was just at the tower and the observation glass shattered," a witness told Newshub in March, when it happened. "Freaked us out, very frightening."

A Sky Tower spokesperson confirmed to Newshub it was only the protective layer that had been damaged and the structural layer beneath remained unharmed.