GRANBURY, Texas -- Forecasters say the tornado that claimed six lives and destroyed dozens of homes in North Texas is believed to have had winds in excess of 320 kph.

The National Weather Service said the preliminary storm estimate for Wednesday night's tornado in Granbury was an EF-4, based on the Fujita tornado damage scale. That means the storm carried wind speeds of 260 kph to 320 kph.

The NWS believes 10 tornadoes raked North Texas in a violent system, including the one in Granbury, about 65 kilometres southwest of Fort Worth. Other tornadoes damaged nearby Cleburne and Millsap.

Forecast Jesse Moore says NWS teams travelled to the affected cities Thursday to survey the damage and speak with emergency management officials.

He says more than one tornado might have touched down in various areas.