There are fears China and Russia could coordinate respective invasions of Taiwan and Ukraine, stretching the military resources of the United States and testing new President Joe Biden.

It comes as tensions mount in Crimea and Ukraine's eastern Donbas region - where Russia keeps deploying more troops - and around Taiwan in the South China Sea, where the US is already responding to an increased Chinese military presence.

The US has warned that China is probably accelerating its plans to capture Taiwan. The two have had separate governments for more than 70 years, but tensions have increased in recent years and China has become "more assertive in sovereignty disputes", AP reports.

Most recently, China shifted an aircraft carrier into disputed waters in the South China Sea after the US moved in with an aircraft carrier and an amphibious assault ship of their own which, among other battleships, are carrying hundreds of US Marines.

Over in Russian-annexed Crimea, President Vladimir Putin continues to amass troops amid sporadic clashes, prompting fears of a sudden escalation.

Simon Tisdall, a foreign affairs commentator, wrote in an opinion piece for The Guardian that while the increased tensions from China and Russia could just be a coincidence, a coordinated effort could push the US to the brink.

He writes that invasions at the same time would be an "Orwellian nightmare" for President Biden.

"While there is no evidence of direct collusion over Ukraine and Taiwan, presidents Putin and Xi are doubtless fully aware of each other's actions, which have an identical, mutually reinforcing effect: putting the wind up Joe Biden's untested US administration," Tisdall said.

"What's now unfolding could be portrayed as the ultimate fulfilment of George Orwell's nightmarish vision, in his dystopian novel, Nineteen Eighty-Four...

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"The Orwellian nightmare for Biden and the west would be a simultaneous Russian invasion of Ukraine and a Chinese attack on Taiwan."

While President Biden has pledged his support to both Ukraine and Taiwan, it's not clear how the US would respond should either fall victim to an attack.

On Monday (NZ time), US Secretary of State Antony Blinken said he had "real concerns about Russia's actions on the borders of Ukraine" and warned of repercussions in an interview with NBC's Meet the Press.

"President Biden's been very clear about this. If Russia acts recklessly, or aggressively, there will be costs, there will be consequences."

Meanwhile Blinken said China were guilty of "increasingly aggressive actions" and promised Taiwan would "have the ability to defend itself" should China attempt an invasion.