TBILISI US Secretary of State Colin Powell, who urged the United Nations to endorse a pre-emptive war to strip Iraq of its weapons of mass destruction, conceded on Saturday that Saddam Hussein’s government may have no longer had such munitions. One day after David Kay, the chief US weapons inspector in Iraq, said he believes Saddam had not stockpiled unconventional weapons for years, Powell told reporters that his prominent February 5 argument was based on ‘‘what our intelligence community believed was credible.’’