Ukraine has warned that the security situation in rebel-held areas in the east has steadily worsened as separatist fighters move closer to demarcation lines separating them from government forces.
A ceasefire agreed in September between rebels and Ukraine's government has in effect been rendered invalid as intense hostilities proceed on a daily basis.
National Security and Defence Council spokesman Andriy Lysenko said that the Russian army is massing troops, including air defence units, near the border. Ukraine accuses Russia of directly supplying separatist forces.
Russian Foreign Ministry spokesman Alexander Lukashevich has reiterated Moscow's denial of that charge.
"There have been and are no military movements across the border or, all the more, any presence of our troops in the south-east of Ukraine," he said.
Ukrainian warnings of potential renewed intense hostilities follow multiple recent observations of large military convoys on the move around separatist-controlled areas. Trucks transporting troops, ammunition, fuel and large-calibre artillery systems have been seen travelling primarily in the direction of Donetsk, the main rebel city.
Ukraine and Nato have said they believe the equipment has been delivered from Russia, although they have yet to provide conclusive evidence for their claims.
Teams from the Organisation for Security and Cooperation in Europe that are monitoring two Russian-Ukrainian frontier crossings have noted an increase in the number of people in military clothing travelling across the border over the past week, however.
"The (observer teams saw) 665 men and women in military-style dress crossing the border in both directions. This is the highest number observed so far," the OSCE said in a statement.
Lysenko, of the National Security and Defence Council, said a surge in hostilities has been registered in Luhansk, one of two regions bordering Russia where separatists have been waging battle for six months.
Government positions in the area have come under repeated rebel rocket and artillery attacks over the previous day, he said.
Ukrainian authorities said residential areas are being increasingly targeted by rebel attacks. Two residents in Zolote, a village in the Luhansk region on the edge of the front line, were hospitalised with shrapnel wounds on Wednesday, Lysenko said.
An area of particular concern to security officials in Kiev is the town of Debaltseve, which lies along the main road linking Donetsk and Luhansk, and is almost entirely surrounded by separatist fighters. It has come under sustained shelling over the past few weeks.
Ukrainian President Petro Poroshenko has pinned hopes for enforcing a truce on teams of monitors from the Organisation for Security and Cooperation in Europe.
On Thursday, he handed over 10 armored vehicles to the OSCE that he said should contribute to ensuring peace is restored.
While Ukraine fights armed separatists in the country's east, it is also battling to get to grips with straitened economic conditions amid galloping inflation.
The central bank said Wednesday that inflation has reached 19.8 percent so far this year and that the national currency, the hryvnia, has lost 59 percent of its value against the dollar over the same period.
"Further devaluation of the hryvnia will lead to additional inflationary pressure, which is projected by the central bank to remain elevated through to the start of next year," the central bank said in a statement.