Light Aircraft Down near Abergavenny
The incident happened at about 15:25 BST between Abergavenny and Raglan.
Two passengers and the pilot have been taken to hospital. Air accident officials have also been alerted.
The road is closed, with two South Wales fire engines at the scene. Crews believed there were about 120 litres of fuel on board, which was contained as they stopped the leak.
From BBC Wales News online