The United Nations said four of its military observers were wounded Saturday when a shell exploded near them in southern Lebanon, where Israel and the Hezbollah movement have traded regular cross-border fire since Israel’s military onslaught in Gaza began last October. Lebanon’s official National News Agency said Israel was behind the explosion, though the United Nations Truce Supervision Organization (UNTSO) has not yet identified the source of the shelling. “This morning four UNTSO military observers on a foot patrol along the Blue Line (demarcating the border with Israel) were injured when …