Russian troops are strengthening their positions in eastern Ukraine and using aid shipments to smuggle in arms and other supplies to separatist forces, Kiev's military said Tuesday.
Military spokesman Andriy Lysenko said units of Russian troops had been identified in the big regional center of Donetsk, towns and villages to its east and in south-east areas near the Sea of Azov.
Fifteen more Ukrainian servicemembers were killed in fighting in the past 24 hours, Lysenko said.
President Petro Poroshenko accused Russia on Monday of "direct and undisguised aggression" in Kiev's five-month war against pro-Russian separatists. He acknowledged the Russian troops had tipped the battlefield balance in favor of the rebels.
Despite the accusations from Kiev and its Western allies, Moscow denies its troops are directly involved.
Military reverses which Kiev blames on Moscow's military intervention include the loss of Novoazovsk, a key town on the Sea of Azov, and the encirclement of forces at Ilovaysk, east of Donetsk.
Ukraine's military on Monday also abandoned control of the main civilian airport near the city of Luhansk, where it said its forces had confronted a Russian tank battalion.
Lysenko said Russian trucks, painted white, were now being used to deliver arms to the rebels.
"Last night four white trucks came ... and after an hour went back again. It's not the first time that white trucks have unlawfully crossed the border accompanied by off-road vehicles and guards," he told journalists.
Vulnerable port city
The loss of Novoazovsk could open a way for the rebels to the major port city of Mariupol, further west along the Sea of Azov coast.
A Reuters reporter there on Tuesday did not hear or see any signs of battle from the city, where Ukrainian forces have been digging defensive trenches for the past few days.
But Lysenko said the separatists had suffered losses, and had withdrawn, after clashing on Monday night with Ukrainian forces near Bezimenne, which lies between Novoazovsk and Mariupol.
Concern continued to be voiced in Kiev over Ukrainian troops apparently encircled by Russian-backed forces in the Ilovaysk area, east of Donetsk.
Kiev's military have clamped a news black-out on information about what is going on there, to avoid endangering a withdrawal of forces there.
But a senior regional official said as many as a hundred Ukrainian service personnel may have been killed. Speaking on his Facebook page, Borys Filatov said: "There are not hundreds of dead. Up to 100. But it tears your heart out."
Serhiy Taruta, the billionaire governor of rebel-held Donetsk, meanwhile warned that no more than a month was left for a negotiated settlement of the conflict in which about 2,600 people have died, according to U.N. estimates. Around 800 of these are members of Ukrainian government forces.
Interviewed by Reuters Television, Taruta, who now runs regional affairs from Mariupol, said: "A huge amount of weapons are unfortunately crossing the Russian border. They (the Russians) bring them to Ukraine to bring death and destruction and they try to annex part of Ukrainian territory.
"So it is very difficult to qualify it any different way as an invasion," Taruta said.
Asked how much time was left to negotiate a settlement, he said: "I think we have no more than a month left."
The chamber of the Ukrainian parliament, which held its first session after the summer recess, reverberated to shrill criticism of Russia.
"It (Russia) has always regarded Ukraine as an appendage. It has always claimed our territory," said Svoboda (Freedom) nationalist leader Oleh Tyahnybok, urging Poroshenko to declare martial law.
Poroshenko and his allies are reluctant to do that. It would mean postponing a parliamentary election called for late October and it might jeopardize disbursement of promised International Monetary Fund credit for Ukraine's hard-pressed economy.