Kiev and the West point their finger at the separatists, saying they may have used a Buk surface-to-air missile supplied by Russia to down the Boeing 777 plane.
But Moscow denies involvement and instead accuses Ukraine’s military.
In a statement, Power referred to a UN Security Council resolution that members including Russia adopted just after the crash. It called for the perpetrators to be held accountable.
However, she stopped short of calling for a UN-backed tribunal to prosecute those responsible.
Britain, France, Malaysia, the Netherlands and others have backed a tribunal, but veto-wielding Security Council member Russia is opposed.
The Netherlands, where most of the victims came from and other countries paid tribute to the victims with memorials Friday.