Washington, Nov 22 : The US Centers for Disease Control and Prevention (CDC) has issued an advisory against cruise ship travels as Covid-19 transmission risk on liners is very high. "For most travellers, cruise ship travel is voluntary and should be rescheduled for a future date," Xinhua news agency quoted the CDC as saying in a statement posted on its website on Saturday. The agency raised its warning to Level 4 from Level 3, citing "very high" Covid-19 risk on cruise ships. It also advised liner passengers to get tested three to four days after the travel and stay at home for seven days even with negative test results. In March, the CDC issued a no-sail order in response to the pandemic. Last month, the agency came up with a framework for a phased resumption of liner operations. Every ship must be certified by the CDC before travelers can board. According to the CDC, from March 1 to September 28, there were "a total of 3,689 confirmed cases or Covid-like illness cases on cruise ships and 412 deaths". The advisory comes as the total number of confirmed coronavirus cases in the US has topped the 12 million mark, according to the Johns Hopkins University. In its latest update on Sunday, the University's Center for Systems Science and Engineering (CSSE) revealed that the country's current caseload and death toll stood at 12,079,296 and 255,804, respectively. By far, the US remains the nation worst hit by the pandemic, with the world's most cases and deaths, making up more than 20 per cent of the global caseload. US Covid-19 cases hit 10 million on November 9, and increased by 1 million within a week. Starting from November 3, the number of daily cases have been surging above the threshold of 100,000, which has never been seen in past months. /IANS