The newly appointed head of Ukraine's navy has sworn allegiance to Crimea in front of the region's unrecognised pro-Russian leader, as Russian forces consolidate their hold on the Black Sea peninsula.

Russia's ambassador to Australia, meantime, has emerged from an official call-up to the Foreign Affairs Department in Canberra warning that his country will act to "ensure the interests" of Russians in Ukraine.

Ukraine has put its military on highest alert, with the defence ministry calling up army reservists, after Russian president Vladimir Putin declared he had the right to invade the country in Moscow's biggest confrontation with the West since the Cold War.

On Sunday, they surrounded several small Ukrainian military outposts and demanded the Ukrainian troops disarm. Some refused, leading to standoffs, although no shots were fired.

All eyes are now on whether Russia makes a military move in predominantly Russian-speaking eastern Ukraine, where pro-Moscow demonstrators have marched and raised Russian flags over public buildings in several cities in the last two days.

Seven things to know about Crimea The Crimean Peninsula is rich in arable land and occupies a strategically important location on the Black Sea.

The Crimean Peninsula is rich in arable land and occupies a strategically important location on the Black Sea. Crimea has a population of 2.3 million, 58 per cent of whom speak Russian and identify themselves as ethnic Russians.

Crimea has a population of 2.3 million, 58 per cent of whom speak Russian and identify themselves as ethnic Russians. The Soviet Union transferred authority over Crimea to Ukraine in 1954.

The Soviet Union transferred authority over Crimea to Ukraine in 1954. Ukraine retained control after the collapse of the USSR in a 1994 agreement brokered by the US, UK and France.

Ukraine retained control after the collapse of the USSR in a 1994 agreement brokered by the US, UK and France. Russia's major naval base is located in Sevastopol and is the base for their Black Sea Fleet. Russia's lease on the base expires in 2042.

Russia's major naval base is located in Sevastopol and is the base for their Black Sea Fleet. Russia's lease on the base expires in 2042. The lease states that Russian personnel may not remove military equipment or vehicles outside the base without permission from Ukraine.

The lease states that Russian personnel may not remove military equipment or vehicles outside the base without permission from Ukraine. The region was a stronghold for ousted president Viktor Yanukovych.

Meanwhile, Ukraine has launched a treason case against its head of navy, Denis Berezovsky, after he surrendered his headquarters at the port of Sevastopol on Sunday on his second day on the job.

The declaration came as Kiev authorities appeared to be losing control of the Russian-speaking Crimean peninsula, which has plunged further and further into disarray since the ousting of Kremlin-backed president Viktor Yanukovych last weekend.

"I swear to execute the orders of the [pro-Russia] commander-in-chief of the Autonomous Republic of Crimea," Rear Admiral Berezovsky said in a televised statement from inside the Crimean headquarters of the Russian Black Sea Fleet, adding that he "swears allegiance to the residents of the Autonomous Republic of Crimea".

It is unclear why and how Rear Admiral Berezovsky switched his allegiance, but Crimea's newly appointed pro-Russia prime minister Sergiy Aksyonov, who is not recognised by Kiev, said the announcement was a "historic event" at a joint press conference with the navy chief.

Troops surround Crimea base in Ukraine

Meanwhile, Ukrainian troops had a tense standoff with hundreds of armed men, believed to be under Russia's orders, who surrounded their military base in Crimea and demanded they surrender.

Ukrainian officials said the armed men had arrived at the base near the village of Perevalne, which hosts the 36th Coast Guard Brigade, in the early morning Sunday (local time).

Caught up in Kiev unrest A Ukrainian couple visiting Brisbane speak about their anxiety over their son's involvement in violent protests in Kiev.

"The [Ukrainian] troops were given an ultimatum to lay down arms, leave their posts and open the gates. The servicemen refused to violate their oath," a local defence ministry spokesman told Ukrainian television.

The defence ministry said 1,000 armed fighters and around 20 trucks were outside the base near the small village, which is about 30 kilometres south-east of Crimea's regional capital of Simferopol.

At the Perevalne base, three Ukrainian soldiers wearing traditional fur caps were standing guard behind the black gate blocking the entrance to the base, a Ukrainian flag still flying outside.

The standoff was tense but peaceful, with curious residents gathering around, some taking pictures with the men surrounding the base.

The standoff is one of several reported across the peninsula since Moscow approved plans to dispatch troops to Crimea on Saturday, sparking fears of a full-scale Russian invasion.

US threatens to isolate Russia with trade, visa bans

Sorry, this video has expired Europe correspondent Mary Gearin reports from London

US secretary of state John Kerry has outlined a host of economic sanctions that are being considered for what he called Russia's "brazen act of aggression" in Ukraine.

Mr Kerry said Moscow still had the means to defuse the crisis and warned that G8 countries and other nations were prepared to "go to the hilt and isolate Russia".

"You don't just, in the 21st century, behave in 19th century fashion by invading another country on completely trumped-up pretext," Mr Kerry said.

"[G8 countries] are prepared to isolate Russia economically. The rouble is already going down. Russia has major economic challenges," he said, mentioning visa bans, asset freezes and trade isolation as possible steps.

On his Twitter account, Mr Kerry said he would travel to Kiev for discussions.

I will be headed to Kyiv, Ukraine tomorrow night for discussions there Tuesday. — John Kerry (@JohnKerry) March 2, 2014

NATO's secretary-general Anders Fogh Rasmussen said the situation needed to be "de-escalated".

Sorry, this video has expired Putin 'staking his reputation' on Ukraine, say Ukrainian studies expert ( Michael Rowland and Virginia Trioli )

Speaking before chairing an emergency meeting of ambassadors from the 28 NATO member states, known as the North Atlantic Council, he warned Russia's actions were unacceptable and could destabilise the continent.

"What Russia is doing now in Ukraine violates the principles of the United Nations charter," Mr Rasmussen told reporters.

"It threatens peace and security in Europe. Russia must stop its military activities and its threats."

Despite the strong words, diplomats said they did not expect NATO to agree any significant measures to pressure Russia, with the West struggling to come up with a forthright response that does not risk pushing the region closer to military conflict.

Ukraine is heavily dependent on Russia for its energy supply. More than half of the 55 billion cubic meters of gas consumed by Ukraine each year is imported from Russia.

Previous disputes over gas prices between Russia and Ukraine in the winters of 2006 and 2009 led to stoppages in exports to Europe at times when consumption demand was at its peak due to the cold weather.

Federal Government summons Russia's ambassador

Meanwhile, Russian ambassador Vladimir Morozov has met with Foreign Minister Julie Bishop's secretary after he was summoned to discuss the country's involvement in the escalating crisis.

Ms Bishop says Australia unequivocally supports Ukraine's territorial integrity.

"We'll express our concerns, officially, directly through him and also seek to ascertain exactly what the Russian government thinks it's doing," she said.

Labor MP Kelvin Thomson says Australia must consider sanctions against Russia.

"A couple of years ago we entered into a trade deal involving uranium sales to Russia," he said. "I think we might need to revisit that."

The Federal Government is urging Australians in Ukraine to leave.

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