HOUSTON (AP) _ An explosion and fire ripped through the fourth floor of a downtown skyscraper Tuesday afternoon, spreading smoke throughout the 34-story tower and forcing the evacuation of office workers, authorities said.

There were no injuries, but damage to the fourth floor appeared to have been severe, Houston Fire Chief Robert L. Clayton said.

The fire erupted at about 3 p.m. in an area where construction crews were removing asbestos from the office building, Clayton said. Smoke rose up stairwells and elevator shafts to the uppermost floors.

Firefighters searched the entire building and found no one, despite early reports that some office workers were overcome by smoke and trapped in an upper stairwell, said fire department spokesman Mike Warnke.

Warnke said an explosion of unknown size apparently sparked the blaze, but that the exact cause was not yet known. The fire was controlled within 30 minutes, he said.

''Some of the walls were blown out, but we haven't been able to tell yet just how bad the damage is,'' Warnke said.

Authorities said they would look into reports that office workers were unable to activate the building's fire alarm.

The building, Allen Center, has an occupancy of about 1,900 people, Warnke said, but many people had taken the day off because of icy Houston streets and it was not known exactly how many were evacuated.