Power said the U.S. believes the plane was likely downed by an SA-11 missile fired from an area in eastern Ukraine controlled by pro-Russian separatists. She said Russia has provided SA-11s and other heavy weapons to the separatists.
She said a Western reporter earlier reported an SA-11 system in separatist-controlled territory "and separatists were spotted hours before the incident with an SA-11 SAM system close to the site where the plane came down."
"Separatists initially claimed responsibility for shooting down a military transport plane, and claimed responsibility and posted videos that are now being connected to the Malaysian airlines crash," Power said. "Separatist leaders also boasted on social media about shooting down a plane, but later deleted these messages."
"Because of the technical complexity of the SA-11, it is unlikely that the separatists could effectively operate the system without assistance from knowledgeable personnel. Thus, we cannot rule out technical assistance from Russian personnel in operating the systems," she said.
Power said Ukraine also has SA-11 missiles but the United States is not aware of any in the area where the plane was shot down.
She said the downing of the Malaysian airlines jet also follows a pattern of attacks on aircraft by the separatists.
"If indeed Russian-backed separatists were behind this attack on a civilian airliner, they and their backers would have good reason to cover up evidence of their crime," Power told the council. "Thus it is extremely important than an investigation be commenced immediately."
The Security Council called for "a full, thorough and independent international investigation."
A press statement approved by all 15 council members expresses the council's "deepest sympathies and condolences to the families of the victims, and to the people and governments of all those killed in the crash."
Council members stood in a moment of silent tribute to the 298 victims at the start of the emergency council meeting, called by Britain.
The council called for an investigation "in accordance with international civil aviation guidelines and for appropriate accountability." It stressed the need for "immediate access by investigators to the crash site to determine the cause of the incident."
The jetliner was shot down Thursday as it flew high above separatist-held territory.