Almost two-thirds of the 160,000 homes in and around Christchurch have been damaged by Saturday's earthquake, New Zealand's prime minister has said. John Key said many had been damaged beyond repair, and that it might take some time to discover the damage to the region's underground infrastructure.A state of emergency in Christchurch has been extended until Wednesday, and the city centre remains cordoned off. Experts have warned a major aftershock could rock the area in the near future. More than 80 aftershocks have been recorded since Saturday's 7.0-magnitude tremor, the strongest of which had a magnitude of 5.1. "It is still possible that we'll have a magnitude 6 in the next week, and people ought to be aware of that, particularly if they are around structures which are already damaged," Ken Gledhill of the Institute of Geological and Nuclear Sciences told the NZPA news agency. "For a shallow earthquake like this, they will go on for weeks," he added. "And if a building is badly damaged, it won't take much shaking to push it over."Despite the widespread damage caused by the strong earthquake, no-one was killed and only two serious injuries were reported.Mr Key, who visited earthquake-stricken parts of the country's South Island over the weekend, said 430 houses and another 70 buildings had already been earmarked for demolition by assessment teams. About 100,000 of the 160,000 homes in the Christchurch, Selwyn and Waimakariri areas had sustained some damage, he added. "I was awe-struck by the power of the earthquake and the damage it has caused in the city," he told reporters. "It was miraculous that nobody was killed."