Explosions have been heard across Ukraine, just minutes after Russian President Vladimir Putin ordered an attack early on Thursday morning (local time).

Confirmation of Mr Putin’s order came via a televised speech from Moscow at 5am – almost exactly as United Nations Secretary-General António Guterres was speaking in New York with another plea to calm tensions in the region.

“Stop your troops from attacking Ukraine,” Mr Guterres said.

Shortly after Mr Putin’s announcement there were reports of massive explosions in the city of Kramatorsk, in Ukraine’s breakaway Donetsk region. There were also reports of more blasts in Ukraine’s capital Kyiv, its second-largest city of Kharkiv and the Black Sea port city of Odessa.

State-run news agencies RIA-Novosti and TASS said Mr Putin had urged Ukrainian forces to lay down their arms and go home.

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Earlier, Ukrainian President Volodymyr Zelensky said Russia had approved an offensive against his nation. In an address posted to his Facebook account, Mr Zelensky said the trigger for an invasion could come at “any minute”.

“The whole world is saying this can happen any day now. The trigger can appear any minute,” he said, speaking in Russian.

“You are being told this fire will free the Ukrainian people. But Ukrainian people are free.”

Mr Zelensky said he wanted to “address all the Russian citizens”, in an emotional plea to his neighbours.

“Not as a president. I address Russian citizens as a citizen of Ukraine,” he said.

“There are over 2000 kilometres of common border between us. Your army is along that border now. Almost 200,000 soldiers. Thousands of military vehicles.

“Your leadership approved for them to take a step further, to the territory of another country.”

Mr Zelensky concluded: “This step can become a beginning of a great war at the European continent.”

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Mr Putin’s declaration came as the United Nations Security Council held its second emergency meeting on the Russian threat for the week. The US ambassador to the UN, Linda Thomas-Greenfield, confirmed military operations were underway in Donbass.

Also on Thursday (Australian time), separatists in Ukraine asked Moscow to help repel “aggression” and explosions rocked the breakaway eastern city of Donetsk, US Secretary of State Antony Blinken warned the invasion could come “before the night is over”.

Hours after the breakaway regions issued their plea, at least five explosions were heard in the separatist-held eastern Ukraine city of Donetsk early on Thursday morning, a Reuters witness said. Four military trucks were seen heading to the scene.

Elsewhere, a conflict monitor zone has warned airlines should stop flying over any part of Ukraine because of the risk of an unintended shoot-down or a cyber attack targeting air traffic control amid the escalating tensions.

“Russia has positioned its forces at the final point of readiness,” Mr Blinken told NBC Nightly News late on Wednesday (US time).

“Everything we’ve seen over the last 24-48 hours has Russia putting the final touches on having its forces in place across all of Ukraine’s borders, to the north, to the east, to the south, to be ready for a full-on invasion”.

Announcing Australia had joined Western allies in imposing sanctions on Russia on Wednesday, Prime Minister Scott Morrison also said there were reports of a likely full-scale attack within 24 hours.

Pentagon officials also told a briefing Russian troops were moving into disputed breakaway regions.

A Reuters witness has reported seeing convoys of military equipment, including nine tanks, moved towards the eastern Ukrainian area of Donetsk from the direction of the Russian border late on Wednesday (local time).

Shelling has intensified in the east, where Russian President Vladimir Putin has recognised two separatist regions as independent and ordered the deployment of what he called peacekeepers, a move the West calls the start of an invasion.

Moscow has long denied that it has plans to invade, even though Mr Putin has tens of thousands of troops massed on the Ukraine border.

The leaders of the two breakaway areas want Mr Putin to intervene.

“I am asking for help to repel the Ukrainian regime’s military aggression against the population of the Donetsk People’s Republic,” said Denis Pushilin, who heads the area Moscow recognised as independent, according to TASS news agency.

The White House rejected the comments as another Russian false flag operation, a fake crisis manufactured to justify greater Russian military intervention in Ukraine.

“This is an example of it,” White House press secretary Jen Psaki said.

“That is suggesting that they feel under threat. By whom? The Ukrainians that the Russians are threatening to attack?”

Mr Zelensky said Moscow had approved an offensive and not replied to an invitation for talks.

“Today I initiated a telephone conversation with the President of the Russian Federation. The result was silence,” he said.

Satellite imagery taken on Wednesday showed new deployments in western Russia, many within 15 kilometres of the border with Ukraine and less than 80 kilometres rom the Ukrainian city of Kharkiv, a US company Maxar said.

The images showed field deployment, military convoys, artillery and armoured personnel carriers with support equipment and troops. The images could not be independently verified by Reuters.

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A 30-day state of emergency in Ukraine restricting the freedom of movement of conscripted reservists, curbing the media and imposing personal document checks began on Thursday.

The Ukrainian government has also announced compulsory military service for all men of fighting age.

While the West has held off the most stringent sanctions measures it could impose, the US stepped up the pressure by imposing sanctions on the firm building the Nord Stream 2 gas pipeline and its corporate officers.

-with AAP