Ukraine has accused Russia of sending dozens of tanks and other heavy weapons into Ukraine's rebel-controlled eastern areas.
National Security and Defence Council spokesman Andriy Lysenko said at least 32 tanks, 16 artillery systems and 30 trucks loaded with fighters and ammunition had crossed into eastern Ukraine from Russia.
Ukraine and the West have continuously accused Moscow of fuelling a pro-Russian rebellion in the east with troops and weapons, which Russia has denied.
Russia's Defence Ministry had no immediate comment on Mr Lysenko's statement but earlier it again rejected Western allegations that Moscow was deploying more troops near the border.
Ukraine's president Petro Poroshenko has ordered the military to beef up troops on the front line to prevent further losses of territory to the rebels.
Russia's relations with the West have plunged to the lowest point since the Cold War over Moscow's annexation of Crimea and its support for the insurgency in the east.
The US and other western nations have slapped sanctions on Moscow, one of the reasons the Russian ruble has plunged over 40% this year.
Despite a ceasefire signed two months ago, Ukrainian troops and separatist rebels are still fighting near the airport of the main rebel-held city of Donetsk and a few other areas.
Tensions rose further after the rebels held an election on Sunday that Ukraine and the West denounced as a violation of the September 5 truce. Russia welcomed the vote but in carefully chosen language.
Russian president Vladimir Putin's foreign affairs adviser, Yuri Ushakov, emphasised today that Moscow's statement saying it "respects" the rebel vote does not amount to its recognition.
Mr Ushakov added that Russia wants peace talks to continue.
Hostilities appear to have intensified particularly in the Luhansk region, where rebels have made some gradual gains in recent weeks.
Mr Lysenko said several villages in the region have come under sustained rebel fire from multiple rocket launchers and artillery. Five government servicemen were killed and a further 16 injured during the previous day's fighting, he said.
Mr Poroshenko's office said in a statement that the Ukrainian leader had discussed the situation in the east with German chancellor Angel Merkel in a telephone conversation.
He told Ms Merkel that conditions of the September 5 truce are being increasingly flouted, which could lead to an escalation in the conflict.
He also complained that Moscow has dispatched another humanitarian convoy into rebel-held regions without prior inspections by Ukrainian border officials or co-ordination with the International Committee of the Red Cross, the statement said.