WASHINGTON— US President Barack Obama raised the stakes Tuesday in a tense standoff with Pakistan, insisting that a detained U.S. Embassy employee who killed two Pakistanis must be freed and sending a high-profile envoy to carry the message that Pakistan has much to lose if the case should drag on.Obama insisted the "simple principle" of diplomatic immunity meant that Pakistan must release the 36-year-old US official, Raymond Allen Davis. Davis has been held since the shootings almost three weeks ago."If it starts being fair game on our ambassadors around the world, including in dangerous places where we may have differences with those governments ... that's untenable," Obama said at a news conference, his first public remarks on the case. "It means they can't do their job. And that's why we respect these conventions and every country should as well."The Davis case has become a flash point for Pakistani nationalism and anti-American suspicion, which makes it harder for Pakistani authorities to back down despite intense US pressure.Thousands have rallied to demand that Davis be hanged, and the Taliban have threatened attacks against Pakistani officials who might be involved in freeing the American.