Mechanical failure is being viewed preliminarily as the primary theory of a Kolavia Airbus A321 crash on the Sinai Peninsula, a source with knowledge of the situation told Interfax on Saturday.

"Mechanical failure on board the aircraft is being viewed preliminarily as the primary theory," the source said.

Among other possible causes of the crash analyzed by specialists are the human factor and poor weather conditions combined with complicated mountainous terrain, he said.