KABUL, Afghanistan, June 5 — The Afghan Defense Ministry said Tuesday that a boat full of fleeing Taliban fighters had been fired upon by NATO helicopters and sunk in the Helmand River, in southern Afghanistan. All on board, between 20 and 30 guerrillas, drowned, said Gen. Zaher Azimi, a ministry spokesman. It was the second sinking on the river reported in recent days. Another boat, carrying about 60 people, sank Saturday in the same area, although there were conflicting reports as to whether those aboard had been civilians or Taliban fighters. General Azimi said that in Tuesday’s episode, NATO helicopters...