Sri Lanka was under new pressure Friday to submit to a war crimes investigation after the United Nations authenticated a video allegedly showing prisoners being executed by troops last year.The government in Colombo has faced repeated calls for a probe from Western countries and rights groups ever since it launched a final offensive to crush Tamil Tiger separatist rebels in the country's northeast in early 2009. The assault brought an end to a 37-year-old ethnic conflict that had claimed 100,000 lives since 1972, but hurt the reputation of the country abroad amid criticism of the conduct and tactics of government forces. The UN estimates that 7,000 civilians perished during the first four months of last year, though this figure is disputed by the government.