RUSSIAN President Vladimir Putin has called on pro-Moscow separatists in Ukraine to open a “humanitarian corridor” to allow Ukrainian troops to escape from the rebel-held eastern town of Novoazovsk.

“I call on the rebel forces to open a humanitarian corridor for the Ukrainian troops who are surrounded, so as to avoid unnecessary casualties and to give them the opportunity to withdraw from the zone of operations,” Putin said in a statement issued overnight.

Earlier today, Tony Abbott branded Russia’s latest actions in Ukraine an invasion and said it’s “completely, absolutely and utterly unacceptable”.

But the Prime Minister said Australia could make a unilateral decision to exclude Putin from the G20 leaders’ summit in Brisbane in November.

NATO said at least 1000 Russian troops had crossed the Ukraine border and are now helping pro-Kremlin separatists who have been fighting against Kiev’s rule since April.

“If, as seems to have been the case, Russian armed forces have simply moved across the border, that is an invasion,” Mr Abbott told reporters in Canberra this afternoon.

“And it is utterly reprehensible.

“It is an absolutely clear-cut case of a larger country bullying a smaller country and this should have no place in our world.”

Russia’s latest actions are likely to intensify calls that Mr Putin be excluded from the G20 talks.

But Mr Abbott says he is still weighing his options.

“It’s not a decision which Australia really has a right to make unilaterally,” he said.

Opposition Leader Bill Shorten said Mr Abbott and Foreign Minister Julie Bishop should now be talking to other G20 nations about the possibility of excluding Mr Putin.

“It is an international conference, not an Australian Conference. I understand that,” he said.

“But following the tragic events of MH17, the complete lack of remorse or responsibility, the lack of full cooperation in terms of MH17, and now scenes of thousands of Russian soldiers flooding across the Ukrainian border, I think most Australians have grave reservations about welcoming Mr Putin to Australia.”

The government should also consider ratcheting up its sanctions against Russia, Mr Shorten said.

He echoed Mr Abbott’s comments that Russia’s latest actions were “reprehensible” and amounted to an invasion.

His comments come after US President Barack Obama said “it’s plain for the world to see” that Russian forces are fighting in Ukraine, but ruled out American military action in the war-torn country.

On Thursday, Obama said “Russia is responsible for the violence in eastern Ukraine”, adding that German Chancellor Angela Merkel agreed on this assessment during a recent conversation.

“The violence is encouraged by Russia, the separatists are trained by Russia, they are armed by Russia, they are funded by Russia,” Mr Obama said.

“Russia has deliberately and repeatedly violated the sovereignty and territorial integrity of Ukraine.”

However, Mr Obama said America’s commitment to defend NATO states did not extend to non-alliance member Ukraine.

“We are not taking military action to solve the Ukrainian problem,” Mr Obama said.

“Ukraine is not a member of NATO. But a number of those states that are close by are. And we take our Article Five commitments to defend each other very seriously.”

He told reporters that he would host Ukrainian President Petro Poroshenko in September to discuss the escalating crisis. The meeting will be on September 18, the White House said.

Mr Obama’s comments came after a top UN official told the Security Council that further violence in Ukraine marked a dangerous escalation, but said the UN had no way of verifying the latest reports of a Russian invasion.

In an emergency session on the growing crisis in Ukraine, members expressed outrage, with US Ambassador Samantha Power saying Russia “has manipulated. It has obfuscated. It has outright lied.”

Russian Ambassador Vitaly Churkin offered a spirited defence, saying Kiev “is waging war against its own people.”

The emergency Security Council meeting came hours after a top Ukrainian official said two columns of Russian tanks and military vehicles fired missiles from Russia at a Ukraine border post, then rolled into the country.

That opened a new front in the war in eastern Ukraine between pro-Russia separatists and the new government of President Petro Poroshenko.

Statements from NATO, Mr Poroshenko, the separatists, the United States and the president of the Security Council left no doubt that Russia had invaded Ukraine.

A top NATO official said Russian troops have entered Ukraine with sophisticated equipment and have been in direct “contact” with Ukrainian soldiers, resulting in casualties.

The new south-eastern front raised fears that the separatists are seeking to create a land link between Russia and Crimea, which Russia annexed in March.

UN Undersecretary-General of Political Affairs Jeffrey Feltman opened the meeting telling council members the latest developments mark a “dangerous escalation in the conflict.”

Ms Power reminded the council that the meeting was “the 24th session to try to rein in Russia’s aggressive acts in the Ukraine.”

“Every single one has sent a straightforward, unified message: ‘Russia, stop this conflict. Russia is not listening’,’’ she said, adding that Russia’s force along the border is the largest it’s been since it started deploying in late May.

“A Russian soldier who chooses to fight in Ukraine on summer break is still a Russian soldier,” she said.

Mr Churkin, for his part, did not deny the presence of Russian soldiers in Ukraine.

“There are Russian volunteers in eastern parts of Ukraine. No one is hiding that,” he said.

But he questioned the presence of Western advisers and asked where Ukrainian troops were getting weapons.

Mr Churkin said he wanted to “send a message to Washington: Stop interfering in the internal affairs of sovereign states.”

Lithuania’s UN ambassador, Raimonda Murmokaite, who requested the emergency meeting, tweeted prior to its start: “An invasion is an invasion.”

Earlier, Ukraine President Mr Poroshenko confirmed Russian regular forces have directly invaded the war-torn east part of the country, and called an emergency meeting of Ukraine’s security and defence council.

The US ambassador in Kiev also declared Russia was “directly involved” in fighting in the war-torn east of Ukraine.

“An increasing number of Russian troops are intervening directly in fighting in Ukrainian territory,” Geoffrey Pyatt wrote on Twitter, adding that Moscow was “directly involved in the fighting” and had sent in its “newest air defence systems including the SA-22”.

A senior NATO official says that “well over a thousand” Russian troops are operating inside Ukraine.

“They support separatists, fighting with them and fighting among them,” the official said on Thursday on condition of anonymity, adding that the supply of arms by Russia had increased in both “volume and quality”.

The official, who was speaking to reporters ahead of a NATO summit next week in Britain, said the situation was made even more worrying because the key route between Donetsk and Novoazovsk, on the Sea of Azov close to the Russian border, had been cut off by pro-Kremlin forces.

“The supply line is cut” for the Ukrainian army, he said.

The official warned that the latest events in Ukraine “have made clear that the security paradigm in Europe has fundamentally changed” in the face of a “very aggressive Russia”.

He said the past weeks have seen a “real upsurge in Russia’s activity” in the flashpoint region, including the supply of weapons, ammunition, special forces training, intelligence and logistic support.

“All this has been systematically denied, adding confusion,” he said.

The United Nations announced it would hold an emergency meeting in New York overnight to deal with the crisis.

British Prime Minister David Cameron has warned Russia it could face “further consequences” following reports Moscow had sent troops into eastern Ukraine, which he said would be “completely unacceptable and illegal”.

“We urge Russia to pursue a different path and to find a political solution to this crisis. If Russia does not, then she should be in no doubt that there will be further consequences,” Cameron said in a statement overnight.

“President Putin has said that Russia is willing to find a peaceful solution to the conflict but this is not credible when Russia is supporting pro-Russian separatists in Ukraine with arms and troops,” Cameron said.

“It is simply not enough to engage in talks in Minsk, while Russian tanks continue to roll over the border into Ukraine,” he said, referring to talks this week between Russian President Vladimir Putin and his Ukrainian counterpart Petro Poroshenko.

Ukraine’s ambassador to the EU immediately called for “large-scale” military assistance from Brussels as the Russian troops were fighting in the east of the country.

Kostiantyn Yelisieiev called for EU leaders who are meeting Saturday to decide on “further resolute significant sanctions and large-scale military and technical assistance to Ukraine in order to stop the aggressor”.

The russians regular troops invaded in Ukraine! This is need to stop immediately!!! #UkraineUnderAttack — Ледокоин Гетьмана (@YevhenS) August 27, 2014

The European Union said it was “extremely concerned” by reports of an incursion by Russian troops into Ukraine.

“We are extremely concerned by the latest developments, including reports on what is happening on the ground,” Maja Kocijancic, spokeswoman for EU foreign policy chief Catherine Ashton, said at a news conference.

Ukraine’s ambassador to the OSCE accused Russia of a “direct invasion”.

“We registered a direct invasion by the Russian military into the eastern regions of Ukraine,” Ihor Prokopchuk told journalists following a special meeting of the European security body to discuss the latest developments in Ukraine.

“The situation has significantly aggravated,” he said in English, citing the capture “by regular Russian forces” of the key south-eastern town of Novoazovsk and several other surrounding towns.

RUSSIAN TROOPS TAKE OVER NOVOAZOVSK AND OTHER LOCALITIES IN SOUTH OF DONETSK REGION - NSDC #RussiainvadedUkraine #UkraineUnderAttack — ЄВРОМАЙДАН (@euromaidan) August 28, 2014

“Ukraine views the latest developments as active aggression against Ukraine,” Prokopchuk said.

German Chancellor Angela Merkel demanded an explanation from Russia’s President Vladimir Putin amid reports that Russian troops have launched an incursion into southeast Ukraine.

Merkel said European leaders would discuss possible new measures against Moscow at a summit in Brussels on Saturday.

“We want a diplomatic solution, and we will not let up on this, but we have to acknowledge that things have become more difficult and worsened again in recent days,” she said.

The BBC’s Barbara Plett Usher in Washington reported that the suspicion is that Moscow is opening a new front to divert Ukrainian forces from the besieged cities of Donetsk and Luhansk, where they have made significant advances against pro-Russian separatists.

Poroshenko said he called the meeting of Ukraine’s security and defence council due to “the rapidly deteriorating situation in Donetsk region, in particular in Amvrosiyivka and Starobesheve, as an invasion of Russian forces has taken place”.

READ POROSHENKO’S REVISED STATEMENT IN FULL

Earlier the security and defence council said the border town of Novoazovsk and other parts of Ukraine’s southeast had fallen under the control of Russian forces who were staging a counteroffensive with rebels.

“A counter-offensive by Russian troops and separatist units is continuing in southeast Ukraine,” the council said in a post on Twitter.

It said Ukrainian government forces had withdrawn from Novoazovsk “to save their lives” and were now reinforcing troops in the port city of Mariupol.

Please go to Russian embassies in your countries and show support to Ukrainians. We really need it.#RussiainvadedUkraine #UkraineUnderAttack — Liza Plitkova (@sycamorrre) August 28, 2014

It added that Russian forces and separatists were combining to launch attacks on Ilovaysk and Shakhtarsk, east of Donetsk.

Ukraine’s Prime Minister Arseniy Yatsenyuk accused Russian President Vladimir Putin of having “deliberately unleashed a war in Europe” and called for an emergency UN Security Council meeting, a called echoed by Lithuania.

Lithuania also accused Russia of a military “invasion” of conflict-torn Ukraine and called for a United Nations Security Council meeting over the issue.

“Lithuania strongly condemns the obvious invasion of the territory of Ukraine by the armed forces of the Russian Federation,” the foreign ministry said in a statement.

“Lithuania urges the UN Security Council to discuss this matter immediately.”

The three Baltic nations, which spent five decades under Soviet occupation until 1991, have been following the Ukraine events closely, concerned about the impact of Moscow’s actions there on their security.

Latvian Foreign Minister Edgars Rinkevics for his part took to Twitter on Thursday to condemn Moscow’s actions.

“Russian invasion in Ukraine must be considered by the UN Security Council as act of aggression, UN must react accordingly, this is war,” he said.

President Vladimir Putin’s spokesman, Dmitry Peskov, has not said the country has invaded Ukraine and Putin himself has made no statement.

The reports echo official denials earlier this year that Russia had sent paramilitary forces into Crimea, and the Soviet Union’s reluctance to acknowledge it was at war in Afghanistan in 1979.

However, wives and mothers of Russian soldiers were set to demonstrate after reports of secret military funerals ratcheted up pressure on Moscow to come clean about its role in the Ukraine conflict.

The women said paratroopers from a base in Kostroma, north of Moscow, were sent on military drills and then went incommunicado, only for some of their husbands and sons to return in pine boxes.

“Cargo-200 arrived in Kostroma yesterday,” Valeria Sokolova, a wife of one of the paratroopers, told AFP, using the Russian military term for body bags.

She said military commanders had refused to confirm that their loved ones had been sent to fight in Ukraine.

“There would only tell us that they are not in Russia,” Sokolova said.