Russia has warned it will respond to what it describes as Nato's "provocative" deployment of soldiers and military hardware near its border.
Nato has been gathering troops in a number of its member states in recent weeks after Vladimir Putin began amassing his own soldiers on the Ukrainian border before launching a full-scale invasion two weeks ago.
Nato members have so far rejected Ukrainian pleas to establish a no-fly zone over its skies, making it clear that any such move could provoke direct engagement with Russian military jets that could in turn spark a wider war.
Troops are currently based in the eastern European countries of Poland, Romania, Slovakia, Latvia, Lithuania and Estonia.
Moscow has frequently accused Nato of provocation, despite repeated assurances that the alliance is a defensive one and poses no threat to Russians. On Wednesday, the Russian foreign ministry stoked tensions further.
"The build-up of NATO forces on the “eastern flank” is openly provocative," it said. "The containment of Russia has obviously become the alliance's main mission again.
"We will respond to the confrontation policy pursued by #NATO towards our country."
Ukraine has stated as recently as last month that joining Nato to bolster its security is a key ambition. Putin is said to be angered by such a move and Russia has stated that Ukraine must "enshrine its neutrality" before any ceasefire can be brokered.
In response, President Volodymyr Zelensky appeared to suggest he would no longer seek membership, telling ABC News on Monday: "I have cooled down regarding this question a long time ago after we understood that ... NATO is not prepared to accept Ukraine."
He added he did not want to be president of a "country which is begging for something on its knees."
Ukraine has also appealed to Nato to supply fighter jets to help with their efforts in holding off Russian forces. On Tuesday, Poland said it was ready to deploy all its MIG-29 jets to Rammstein Air Base in Germany and put them at the disposal of the United States
However, the Pentagon dismissed the proposed solution as "untenable" saying the prospect of the jets' departing from a US and Nato base in Germany "to fly into airspace that is contested with Russia over Ukraine raises serious concerns for the entire Nato alliance".
Putin's actions have now driven 2 million people out of Ukraine and into neighbouring countries as refugees, the UN said on Tuesday.
It is is the largest humanitarian crisis since World War Two.
US intelligence authorities warned on Tuesday that Putin could decide to leave Ukraine if his efforts to take over are continually stalled, but warned he could double down on the violence before deciding to.
“We assess Putin feels aggrieved the West does not give him proper deference and perceives this as a war he cannot afford to lose. But what he might be willing to accept as a victory may change over time, given the significant costs he is incurring,” Avril Haines, the director of national intelligence, told members of the House Intelligence Committee.
Watch: Ukrainian President Volodymyr Zelenskyy addresses MPs in the House of Commons
Western officials believe Putin had hoped to have Ukraine well under his control within days of launching the invasion, but say poor planning, bad leadership and a fierce line of resistance from the Ukrainian people have stalled the Russian progress.
The US defence office has claimed Russia has deployed nearly all the 150,000 troops who were stationed on the border, but has only "made little progress".
An unnamed official added the Russians are "frustrated by a stiff Ukrainian resistance as well as their own internal challenges".
The nation's largest cities are still under Ukrainian control, but coming under constant Russian bombardment as Putin's force step-up their campaign of misery.