India has expressed concern over the high crash rate of its Israeli-made spy drones, taking up the issue with Israeli officials, media reports said Thursday. Four of the 50 Searcher and Heron unmanned aerial vehicles India purchased from Israel have crashed over the last two years, the Press Trust of India news agency quoted Defense Minister Pranab Mukherjee as telling parliament in a written response. According to PTI, Mukherjee said investigations attributed the crashes to systems failure. Indian defense officials have raised the issue with their Israeli counterparts and the manufacturer, Israel Aircraft Industries, has agreed to repair the drones for free, he said. "A continuous and multifaceted effort is under way in the defense forces to enhance and upgrade safety of the UAVs," Mukherjee wrote, adding that measures to enhance the quality of training were also being pursued. In one of the crashes, an Indian air force handler was killed, PTI reported. Israel Aircraft Industries were stunned by the report. "We flatly deny this," said Ya'ir Dubester, director of IAI's Malat division which manufactures the Searcher and Heron. "Not one of our UAVs has ever crashed in India." Analysts have noted that the accident-prone Indian air force has one of the highest crash rates in the world. India is Israel's leading arms buyer and ties between the two countries have warmed considerably this decade. Last year, Israel agreed to sell three Phalcon airborne early warning systems to India. The $1.1 billion deal was Israel's biggest one-time sale of military hardware ever. The Heron UAVs can fly more than 1,000 kilometers at altitudes above 25,000 feet for more than 24 hours, according to IAI.