International pressure on Russia was mounting Thursday over claims it is sending fresh military hardware into eastern Ukraine which could fuel a return to all-out conflict.
After NATO accused Russia of deploying tanks, troops and military hardware to the region, Ukraine said four of its soldiers had been killed in the past 24 hours and 18 wounded.
There were a string of explosions late Wednesday in the rebel stronghold of Donetsk but the situation was calm Thursday, an AFP reporter said, with only occasional exchanges of fire.
Rebels said three people were injured in shelling in Donetsk Wednesday.
A senior Ukrainian security official speaking to AFP on condition of anonymity claimed there were now thousands of Russian troops in the country.
"According to our estimations, there are 8,000 Russian soldiers, maybe more, on our territory at the moment," he said.
The skirmishes on the ground played out against a backdrop of rising Western concern over claims that Russia is dispatching reinforcements to the east of the former Soviet state.
NATO's commander in Europe, US General Philip Breedlove, said Wednesday that "columns of Russian equipment, primarily Russian tanks, Russian artillery, Russian air defence systems and Russian combat troops" were entering Ukraine.
Later, Assistant Secretary-General Jens Anders Toyberg-Frandzen told an emergency meeting of the UN Security Council that it was "deeply concerned" by a possible return to full-scale fighting.
The US Ambassador to the UN, Samantha Power, charged that Russia "talks peace but it keeps fueling war" as Washington warned that the West could ramp up punishing sanctions against Moscow.
But Russia's Deputy Ambassador to the UN, Alexander Pankin, described NATO claims of a Russian military buildup in east Ukraine as a "foray into propaganda".
Pro-Russian separatists in east Ukraine have been fighting Ukrainian forces since April in a war which has claimed more than 4,000 lives and driven hundreds of thousands of people from their homes.
Moscow has repeatedly denied involvement but openly gives political backing to the self-declared separatist statelets in the east.
A fragile ceasefire has been in place for two months and has stopped much frontline fighting although shelling at strategic flashpoints continues.
Kiev warned Wednesday it was preparing for a possible new round of fighting after seeing "increased activity" by Russia and pro-Moscow rebels in the east.
Observers from the Organisation for Security and Cooperation in Europe (OSCE) have reported a number of unmarked military convoys heading towards rebel strongholds in recent days.
The West is watching anxiously to see how the situation in eastern Ukraine will develop as the former Soviet state's harsh winter kicks in.
Toyberg-Frandzen outlined three scenarios -- a "return to full-scale fighting"; a continuation of the current situation "for months" with low-level battles punctuated by periods of increased hostility; or a "frozen" conflict which could draw out the current situation for decades.
The senior Ukrainian security official predicted that pro-Russian forces may try to take control of the entire regions of Donetsk and Lugansk, of which separatists currently only control a part.
They could then try create a corridor to Crimea, which Russia annexed in March, he added.
The Ukraine crisis has sent relations between Russia and the West plummeting to their lowest point since the Cold War.
Russian President Vladimir Putin is likely to face fresh pressure over Ukraine at a G20 summit in Brisbane from Saturday.
Australia said Thursday it was tracking four Russian navy ships including a "heavily armed" cruiser and destroyer, in international waters off its north coast ahead of the high-level meeting.
There is public anger in Australia over the downing of Malaysia Airlines flight MH17 over eastern Ukraine in July, killing 298 people including 38 Australian citizens and residents.
Ukraine, supported by Western nations, accuses Russia of supplying pro-Kremlin separatists with the missile that shot down the airliner but Moscow and the rebels blame Ukrainian forces.
Putin met Australia's Prime Minister Tony Abbott and discussed Ukraine at the Asia-Pacific Economic Cooperation (APEC) summit in Beijing Tuesday.
Abbott had previously threatened to "shirtfront" the Russian president -- an Australian Rules football term in which a player charges an opponent -- over the MH17 disaster.
The Australian premier said Thursday that Russia's naval deployment highlighted its "assertiveness" but was "not unusual" ahead of a major conference.
"We're seeing -- regrettably -- a great deal of Russian assertiveness right now in Ukraine," he added on the sidelines of the East Asia Summit in Myanmar's capital, Naypyidaw.
John Blaxland, an international security expert at the Australian National University, said sending the ships was "huff and puff" by Putin designed to reinforce his "tough, he-man image".