The US and Russia fear that on-board weapons could reach Al Qaeda
U.S. warships surrounded the hijacked MV Faina and its military cargo off Somalia's coast Monday.
Attacks at sea force a UN aid agency to rely on naval guards.
Gunmen hijacked four ship within 48 hours last week. Cash retrieved from ransoms is paying for weapons and salaries of fighters on both sides of Somalia's conflict.
As the violence escalates, observers warn of famine and the worst humanitarian crisis on the African continent.