International aid agencies and governments mobilized on Sunday to respond to the earthquake in Nepal, saying they faced challenges in getting assistance to the country and distributing it amid the widespread devastation there. In the aftermath of the disaster, which has killed more than 2,400 people, injured 5,900 and left many more homeless, development workers said that continued aftershocks, a crippled transport network and the loss of power in parts of the country had made it tough to search for survivors and distribute much-needed supplies.