WASHINGTON, D.C.: Emerging from a meeting with President Bill Clinton on Monday, Admiral Leighton Smith, commander of NATO's peacekeeping mission in Bosnia, said that the deaths on Sunday of four allied soldiers proved that "these threats remain." Three British soldiers were killed when a land mine blew up their vehicle in the town of Mrkonjic Grad; a Swedish soldier died when his armored personnel carrier slid off a road in the north; and an American was grazed by a sniper's bullet in a Sarajevo suburb. It was the deadliest day in NATO's five-week-old mission. "I can't guarantee the American people that we're not going to see casualties because of road traffic accidents or mines or snipers," Smith told reporters outside the White House. "But I can tell you the cooperation we have been seeing tells me we will not see a force-on-force situation." It took more than eleven hours on Monday for NATO soldiers to inch their way through the minefield to the burned-out wreckage of the dead soldiers' Spartan armored personnel carrier. So far eight soldiers have been killed and 35 injured in Operation Joint Endeavor, most of them victims of exploding land mines.