Four suspected gunmen behind the deadliest attack in Russia in 20 years are Tajik citizens
It said in a second statement that the causes had not been disclosed and that any previous comments were not official.
Asked at a briefing in Kiev if the plane could have been downed by a missile, Ukraine's Prime Minister Oleksiy Honcharuk cautioned against speculation until the results of an investigation were known.
He also said that Ukraine had banned flights through Iranian airspace by its airliners from Jan 9.