An improved burner for both liquid and gaseous fuels in which the combustion air is divided into at least two parts, comprising primary and secondary air. The primary air, which is insufficient to completely burn the fuel, creates a flame zone in which there is a deficiency of air, which produces quantities of carbon monoxide and hydrogen. The combustion products are recirculated, cooled and re-entered into the combustion zone in the region of the fuel and primary air entry, resulting in a reduction in the oxides of nitrogen. The secondary air, in quantities greater than the primary air, is introduced into a second combustion zone downstream of the first combustion zone. In the second combustion zone the deficiency in air is made up by the secondary air so that complete combustion can be carried out. If desired, a third quantity of air can be introduced still farther downstream in the flame zone.