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 with the US saying Moscow could begin with aerial bombardments “at any time” more than a dozen nations have urged their citizens to leave Ukraine.
Ukraine’s ambassador in London, Vadym Prystaiko, has backtracked on comments he made to the BBC in which he said Ukraine was willing to be “flexible” on its ambition to join Nato, which would have been be a major concession to Russia.
But in a subsequent interview he said that Ukraine had a constitutional commitment to join Nato and it depended on the “readiness of Nato itself” whether Ukraine would be admitted.
British Armed Forces Minister James Heappey said the UK would support whatever Ukraine decided to do.
Ukraine has made a request via the Organization for Security and Co-operation in Europe (OSCE) for Russia to explain its build-up of troops. Under the Vienna Document, of which Russia is party to, OSCE members can ask for information on a member’s military activities.
“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,” Mr Kuleba 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 had thanked the US for its “unwavering support” and that, at the end, President Zelensky had 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.
In the latest attempt to find a diplomatic solution, German Chancellor Olaf Scholz has meetings scheduled with President Zelensky in Kyiv later 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.
In Washington, President Biden’s National Security Adviser Jake Sullivan said an invasion could begin “any day now”.
Mr Sullivan said the US is closely monitoring for a potential “false flag” operation by Moscow as a pretext for a full-scale invasion so it can claim it is responding to Ukrainian aggression.
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”.