BOA VISTA, Brazil (Reuters) - Firefighters battling infernos in Brazil's northern Amazon appealed for more men and equipment but the arrival of essential water-bombing helicopters was delayed. Fires set by subsistence farmers in the state of Roraima have burned out of control for two months, destroying a vast swathe of highland savannah near the border with Venezuela. Officials say 2.2 million acres (900,000 hectares) of farmland has gone up in smoke, while an unusually severe drought has killed about 20,000 head of cattle. Now the flames are eating into remote rainforest areas which are normally too wet to burn. A column of fire has pushed at least seven miles (11 km) into the Portugal-sized jungle reservation of the primitive Yanomami Indians. Other tribes which live in the savannah, like the Macuxi and Wapixana groups, have seen their crops fail in the dry conditions and are running out of food. Several officers were being treated for respiratory and eye problems caused by dense smoke. Despite its vast forest resources, Brazil has no specialized water-carrying planes or helicopters. Given the scale of the fires, officials were considering a formal request for help to the U.N. Department of Humanitarian Affairs. Environmental officials blame the fire on a six-month drought in the region which has dried out the forest, and on the slash-and-burn techniques of poor farmers who every year set fire to their plots to fertilize the thin soil with ash. Meteorologists say Roraima would normally be receiving the first showers of the rainy season by now. But the El Nino weather phenomenon has altered the usual patterns and the forecast is for no rain until late April.