MORE than 200 police have responded to reports of a gunman in a Houston, Texas, hospital. But no evidence of a shooter - or a shooting - has been found.

The police, some in body armour and armed with assault rifles, formed a perimeter around the Ben Taub Hospital's grounds before moving floor-to-floor to evacuate patients and staff.

They were responding to reports of 'shots fired' on the second floor of the complex, though nobody appears to have been injured.

Hundreds of staff and patients were evacuated under police guard.

"All patients and employees are safe at this point," the Houston police department chief has told reporters.

"I am confident that if there was a threat, that threat is not here now," he said.

A police sweep of the six-floor hospital and grounds followed reports the gunman was believed to be 'holed up' on the building's third floor. But at a recent press conference, Houston police say they have found no trace of the reported gunman.

Nor did the find any signs of gunfire.

SWAT police and sniffer dogs continue to scour the site in the search for a suspect described as a middle-aged, bald white man.

"I heard them screaming 'code white,'" Edward Jaime told ABC13 Eyewitness News. A "code white" is enforced to tell employees not at the hospital to stay away from the area.

It's the second such incident at the hospital in recent months.

Houston police were involved in a shooting inside Ben Taub Hospital on October 5, 2016, when a deputy was forced to shoot a suspect on the building's fifth floor.

That suspect was in custody for a probation revocation hearing and was brought to the hospital after complaining about seizures.