According to a report in The New York Times (read here), Russia has built up 17 battalions within a few miles of the Ukrainian border and has also expanded its firepower and surface to air missiles units to 14.
Intelligence experts say that on the pretext of 'peacekeeping operations' Putin may send his forces across the border in case the Ukrainian government tried to regain the territory in eastern Ukraine.
Some officials even say that more the Ukrainian forces succeed , it will add more pressure on Putin to escalate the matter. Several American intelligence agencies have confirmed the presence of Russiam armoured vehicles and trucks near the border.
But the Western intelligence had no independent confirmation that they had crossed into Ukraine.