Sanctions that have pinched North Korea's health care system should be eased, a group of Nobel laureates said Saturday, after a rare visit to the nuclear armed state that coincided with its ruling party congress. Embargoes on the flow of goods into the isolated country have squeezed the quality of medical care and research, they said, following visits to hospitals and labs in Pyongyang. "You cannot turn penicillin into a nuclear bomb," Aaron Ciechanover, who won the top prize for chemistry, told a media conference in Beijing a day after returning from the visit. "You don't pressurise via making people sicker," he said: "That's not the right way to go." The three prize winners from Norway, Britain and Israel spent a week in the country on a humanitarian trip organisers said would be an exercise in "silent diplomacy". Their visit came as North Korean leader Kim Jong-un opened the country's first ruling party congress since 1980 by hailing its "magnificent... and thrilling" nuclear weapons programme. World powers have tightened sanctions on the isolated state this year after Pyongyang carried out several ballistic missile launches and its fourth nuclear bomb test -- and analysts predict another could be in the works. While the sanctions do not target medical aide, tough South Korean restrictions have stopped some medicines from reaching its northern neighbour, according to a recent report in the Washington Post. "Many of the things the doctors would like, the professors would like, they just can't have them because of the embargo," said Richard Roberts, who won the prize for medicine. The trio of Nobel prize winners, which also includes economics laureate Finn Kydland, visited a children's hospital, science facilities and a farm, among other sights. The laureates described clean, modern facilities -- a stark contrast to other accounts of the country as brutally impoverished -- and two said they had invited young researchers to work in their labs. The few opportunities for foreigners to visit the country are tightly stage-managed, with the government carefully controlling most interactions with the North Korean people. Planning for the trip began more than two years ago after the Vienna-based International Peace Foundation (IPF) received an unsolicited email from the Korean National Peace Committee. South Korea's government asked the group to postpone the trip when it emerged that it would coincide with the congress, citing fears it could be "misused", IPF chairman Uwe Morawetz said, but scheduling restrictions made it impossible.