White House adviser Tony Blinken says there is evidence that Russia is preparing to deliver more powerful rocket launchers to separatist rebels in eastern Ukraine.
Western powers have accused pro-Russian rebels in the area of shooting down Malaysia Airlines flight MH17 with a rocket.
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The memorial service will be attended by Prince Harry and Meghan Markle.
David Browning admits manslaughter by diminished responsibility but denies murder.
Four people have died and the suspect is still at large, police said.