BRISBANE: Australia said Thursday it is tracking four Russian navy ships off its north coast, a move Prime Minister Tony Abbott said was a sign of Moscow’s military assertiveness with relations strained after the downing of Flight MH17 in Ukraine.

The appearance of the fleet coincides with Vladimir Putin’s arrival at the G20 summit in Brisbane this weekend and follows a highly anticipated exchange between the Russian leader and Australian Premier Tony Abbott in Beijing.

Putin’s presence in Brisbane was questioned given Australia’s anger at the downing of Malaysia Airlines flight MH17 in July, in which 298 people died in rebel-held eastern Ukraine, including 38 Australian citizens and residents.

But the G20's consensus was that he should attend despite Kiev and the West claiming the plane was blown out of the sky with a missile supplied by Russia, an allegation Moscow denies.

International pressure on Russia is mounting over claims it is sending fresh military hardware into eastern Ukraine which could fuel a return to all-out conflict.

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 defense 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 cease-fire 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 Organization for Security and Cooperation in Europe (OSCE) have reported a number of unmarked military convoys heading toward 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.