In the summer of 1962, Mao Zedong agreed to supply Hanoi with 90,000 rifles and guns free of charge, and starting in 1965, China began sending anti-aircraft units and engineering battalions to North Vietnam to repair the damage caused by American bombing. In particular, they helped man anti-aircraft batteries, rebuild roads and railroads, transport supplies, and perform other engineering works. This freed North Vietnamese army units for combat in the South. China sent 320,000 troops and annual arms shipments worth $180 million. The Chinese military claims to have caused 38% of American air losses in the war.