Ukraine appealed for NATO's help on Wednesday after reporting that a huge convoy of tanks and weaponry from Russia was moving through the southeast of the country.
Kiev's claims came just hours after the first meeting in three months between President Petro Poroshenko and Russian counterpart Vladimir Putin failed to achieve any concrete breakthrough despite talk of a peace road map.
The military said a convoy of up to 100 tanks, armored vehicles and rocket launchers was seen traveling towards Telmanove, a town about 80 kilometers south of the main rebel-held city of Donetsk and 20 kilometers from the Russian border.
Moscow was forced to admit Tuesday that a number of its troops captured by Ukrainian forces had crossed the border "by accident" but continues to deny arming the pro-Kremlin separatists who have been fighting Kiev's rule for four months in an increasingly bloody conflict.
Ukraine has accused its former masters in Moscow of sending in weapons to try to open another front in areas south of Donetsk after Kiev government forces made major advances against the rebels.
Poroshenko and Putin held marathon talks in the Belarus capital Minsk on Tuesday, alongside top EU officials, to try to find ways to end fighting that has now claimed over 2,200 lives in Ukraine's eastern industrial heartland.
Poroshenko said all sides had "without exception" agreed to his peace plan, but demanded "decisive actions which can help bring peace to Ukraine".
He said he and Putin discussed the "necessity of closing Ukraine's borders" to prevent the movement of "equipment, mercenaries, and ammunition".
But Putin again insisted Russia had no role in the conflict, despite accusations by Kiev and the West that it is acting to further destabilize its western neighbor after the annexation of Crimea in March.
"We cannot discuss any ceasefire conditions," he said. "It's not our business, it's Ukraine's internal business."
Putin also sought to play down the capture of 10 Russian paratroopers about 20 kilometers from the border after Russian military sources said they had crossed the border by accident.
In another move set to to raise tensions, Ukraine's Prime Minister Arseniy Yatsenyuk called on NATO to act to help Kiev when the alliance holds a summit in Wales next week.
"We expect our Western partners and the alliance to provide practical help and take crucial decisions at the summit in September," Yatsenyuk said.
Poroshenko is expected at the summit to discuss the crisis with US President Barack Obama.
Russia, increasingly isolated by the West and its economy struggling in the face of punishing EU and US sanctions, has not been invited to the September 4-5 meeting and is vehemently against closer ties between Kiev and NATO.
NATO's chief Anders Fogh Rasmussen said in an interview published Wednesday that the alliance was preparing a rapid response to the standoff that would allow it to deploy troops swiftly to reinforce eastern Europe.
Rasmussen has repeatedly accused Russia of troop buildups on the border and said a unilateral convoy of aid send to the war-torn east last week was a "blatant breach" of Ukraine's sovereignty.
A Ukrainian military source said the latest convoys of armor rolling across the southeast were believed to be Russian.
"You cannot buy 100 tanks at a market in Donetsk or Lugansk," the source told AFP.
"Of course they have been moved from across the border," he said, but added that the military is working on obtaining proof before making a formal accusation.
AFP journalists traveling on the same road north of Telmanove said they had seen tank tracks and heard explosions.
"Last night there was shelling all around," said an elderly woman selling vegetables on the roadside in Rozdolne village, about 25 kilometers north of Telmanove.
Ukrainian security spokesman Andriy Lysenko also said militants and "Russian occupants" had occupied Starobesheve, a town north of Telmanove, along with a string of villages just north of Novoazovsk, a town on the Azov Sea where clashes have been raging for days.
Kiev says 13 soldiers have died in the past 24 hours, while three civilians were killed in Donetsk, according to the city authorities.
British Prime Minister David Cameron said earlier this month that a "robust" defensive presence in eastern Europe was needed and that "we must... review our long-term relationship with Russia."