Insurgent attacks and bombs have killed another four US soldiers in Afghanistan, the NATO-run International Security Assistance force said Monday. Two were killed by a roadside bomb and a third died in a militant attack on Sunday. A fourth soldier was killed by a improvised explosive device on Monday, ISAF said in a statement. "Four ISAF service members died in the last 24 hours in Afghanistan," the statement said. All the casualties were US nationals, it added. The latest deaths took to 481 the number of foreign troops killed in violence in Afghanistan this year, according to an AFP tally based on the independent www.icasualties.org which counts losses in Iraq and Afghanistan. More than half this year's fatalities are US nationals.