Ukraine has called for a meeting with Russia and other members of a key European security group over the escalating tensions on its border.
Foreign Minister Dmytro Kuleba said Russia had ignored formal requests to explain the build-up of troops.
He said the "next step" was requesting a meeting within the next 48 hours for "transparency" about Russia's plans.
Russia has denied any plans to invade Ukraine despite the build-up of some 100,000 soldiers on Ukraine's borders.
But some Western nations have warned that Russia is preparing for military action, with the US saying it could begin with aerial bombardments "at any time".
More than a dozen nations have urged their citizens to leave Ukraine, and some have pulled embassy staff from the capital. CBS News reported that the US is preparing to withdraw all its personnel from Kiev within the next 48 hours, citing three sources.
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Mr Kuleba said Ukraine had, on Friday, demanded answers from Russia about their intentions under the rules of the Vienna Document, an agreement about security issues adopted by the members of the Organisation for Security and Cooperation in Europe (OSCE), which includes Russia.
"If Russia is serious when it talks about the indivisibility of security in the OSCE space, it must fulfil its commitment to military transparency in order to de-escalate tensions and enhance security for all," he said.
However, Ukraine's President Volodymyr Zelensky, who criticised the "panic" that could spread from such claims, said he had seen no proof that Russia was planning an invasion in the coming days.
On Sunday, he spoke for nearly an hour by phone with US President Joe Biden. The White House said President Biden had reiterated US support for Ukraine, and that both leaders had agreed on "the importance of continuing to pursue diplomacy and deterrence".
Ukraine's statement of the call said its president thanked the US for its "unwavering support" and that, at the end, President Zelensky invited the US leader to come to Ukraine. There has been no comment on the invite from the White House.
An hour-long call between President Biden and Russian leader Vladimir Putin the day before failed to yield a breakthrough.
Diplomatic efforts are set to continue into Monday. German Chancellor Olaf Scholz has meetings scheduled with President Zelensky in Kyiv on Monday and with President Putin in Moscow on Tuesday.
The chancellor, who took over the leadership of Germany from Angela Merkel in December, has warned of severe economic consequences for Russia if it should launch any invasion, echoing statements by other Western nations and members of the Nato military alliance.
But Berlin officials have downplayed any expectation of a breakthrough.
Meanwhile, UK Prime Minister Boris Johnson plans to hold fresh diplomatic talks across Europe to bring Russia "back from the brink" of war.
Comments made by Mr Johnson's defence secretary earlier on Sunday drew ire from Ukraine's ambassador to the UK, Vadym Prystaiko.
Ben Wallace compared the current situation to the appeasement of Nazi Germany in the run-up to World War Two in an interview with the UK's Sunday Times newspaper.
Mr Prystaiko responded by telling the BBC's Broadcasting House radio programme: "It's not the best time for us to offend our partners in the world, reminding them of this act which actually did not bring peace but the opposite – it bought war."
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Mr Prystaiko later acknowledged to the BBC that his country may be forced to contemplate shelving any potential ambitions of joining Nato if they were "threatened… and pushed to it".
One of Russia's demands is that Ukraine – not currently a member – would never be allowed to join Nato.
Nato and Western nations insist that sovereign countries such as Ukraine are free to decide things for themselves, including potentially applying for membership of the security alliance, formed in part as a counter-measure to the Soviet Union in the aftermath of World War Two.
Russia contends that its build-up of troops along the Ukraine border is its own concern, within its own territory. On Sunday, senior foreign policy official Yuri Ushakov characterised the US warnings of imminent invasion as "hysteria has reached its peak".
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