New U.S. intelligence assessments indicate that Moscow likely provided pro-Russia separatists in eastern Ukraine with sophisticated antiaircraft systems in recent days, matching evidence put forward by Ukraine and bolstering charges that Russia was the source of the weapon that shot down Malaysia Airlines Flight 17 this week, killing 298.

U.S. officials say they now suspect that Russia supplied the rebels with multiple SA-11 antiaircraft systems by smuggling them into eastern Ukraine with other military equipment, including...