UPDATE 3.25pm: The barn was well alight when crews arrived and they tackled the blaze for more than three hours, says head of control for South Wales Fire and Rescue Service, Jennie Griffiths on Twitter.
The barn contained cylinders which needed to be cooled by crews in order to make them safe from explosions, she said.
The barn, six metres in height and 25 metres squared, was used to store building materials and vehicles. Fire fighters used four ground monitors, three jets and four breathing apparatus were used.
They left just after 2pm.
UPDATE 2.30pm: No one is trapped and there are no injuries or casualties, a spokeswoman for South Wales Fire and Rescue Service has confirmed.
UPDATE 1.46pm: Fire fighters have surrounded the fire at the farm on Graig Road and are tackling it with three pumps and two water bowsers.
A spokeswoman for South Wales Fire and Rescue Service said a residential building is involved in the fire as well as the barn but could not give any further information.
12:39pm: A BARN and several other buildings are on fire in Penallt, Monmouthshire.
Fire crews from Monmouth, Chepstow, Cwmbran, Hereford and Worcester are at the scene.
A spokeswoman for South Wales Fire and Rescue Service said they are very busy at the moment but will be releasing details shortly.