Plans to rescue nine copper miners trapped in a caved-in shaft in southern Peru suffered a setback that could delay their release until Wednesday, Mining and Energy Minister Jorge Merino said. "The best guess is it will be tomorrow (Wednesday) early," said Merino. He cautioned against a swifter rescue operation, saying there was a risk of further cave-ins and collapses inside the Cabeza de Negro mine, 325 kilometers (200 miles) south of Lima. The miners have been trapped for six days in a tunnel 250 meters underground in a horizontal tunnel. Some of them are reporting health problems, such as stomach disorders. Rescuers have closed the distance to reach the miners to as little as two or three meters but have been blocked by rocks and soil that require additional shoring up. President Ollanta Humala visited the mine late Tuesday. "There have been some setbacks caused by the structure of a rock," Humala said. "We have to wait a few hours more." The delay was necessary "so that there are no victims," he said. More than 40 rescue workers were pressing ahead with the effort. Workers specializing in reinforcing tunnels arrived Monday to shore up shafts at the partly collapsed mine. The mine workers, aged 22 to 59 and including a father and son, were not injured and remain together. Trapped since Thursday, they were being supplied with oxygen, water and soup through a metal tube. Cabeza de Negro is an unlicensed mine that was abandoned more than two decades ago by its owners, but continues to be exploited. With international market prices for metals high, informal "wildcat" mining has been on the rise in recent years in Peru, one of the largest producers of silver, copper and gold. The Peruvian miners' fate recalled a similar case in Chile that made world headlines. In August 2010, 33 miners were trapped after a cave-in in the San Jose gold and copper mine in northern Chile -- after 69 days and a spectacular rescue operation with the world watching, they were brought out safely.