Ukraine has called for a meeting with Russia and other members of the OSCE group on the escalating tensions on its borders.
Foreign Minister Dmytro Kuleba said Russia had ignored his country’s official requests to explain its troop increase.
He added that the “next step” is to request a meeting within the next 48 hours for “transparency” regarding Russia’s plans.
Russia has denied any plans to invade Ukraine despite massing some 100,000 troops on Ukraine’s border.
Kuleba said that Ukraine requested answers from Russia on Friday under the rules of the Vienna Document, an agreement on security issues adopted by members of the Organization for Security and Cooperation in Europe, which includes Russia.
“If Russia is serious when it talks regarding the indivisibility of security in the OSCE space, it must fulfill its commitment to military transparency in order to calm tensions and enhance security for all,” he added.
Some Western countries have warned that Russia is preparing for an invasion, and the United States has said Russia might start an aerial bombardment “at any time”.
More than a dozen countries urged their citizens to leave Ukraine, and some withdrew their embassy staff from the capital. CBS News reported that the United States is preparing to withdraw all of its nationals from Kiev within the next 48 hours, citing three sources.
But Ukrainian President Volodymyr Zelensky, who has criticized the “panic” that might spread over such allegations, said he had no evidence that Russia was planning an invasion in the coming days.
On Sunday, President Zelensky spoke for regarding an hour by phone with US President Joe Biden. The White House said President Biden reiterated US support for Ukraine, adding that the two leaders agreed on “the importance of continuing to pursue diplomacy and deterrence.”
Ukraine’s statement regarding the call said that its president thanked the United States for its “steadfast support”, and also invited the US president to visit Ukraine. There was no comment from the White House.
An hour-long call between President Biden and Russian leader Vladimir Putin on Saturday failed to achieve a breakthrough.
Correspondents in Kiev say panic has not spread to the city, but such warnings from the United States and others have had an effect.
Some commercial airlines canceled flights to Ukraine at the weekend, while Ukraine said it would commit hundreds of millions of dollars to maintain aviation safety and security and to insure the work and routes of airlines and companies.
In July 2014, 298 people were killed when a Malaysia Airlines plane was shot down over the eastern part of Ukraine occupied by Russian-backed rebel forces.
In the context of diplomatic efforts, German Chancellor Olaf Schulz is scheduled to hold meetings with President Zelensky in Kiev on Monday and with President Putin in Moscow on Tuesday.
The chancellor, who took over the leadership of Germany from Angela Merkel in December, warned of dire economic consequences for Russia if it launched an invasion, echoing statements by other Western countries and members of the military NATO.
But officials in Berlin played down any expectation of a breakthrough.
Russia is demanding that Ukraine, which is not a member of NATO, never be allowed to join it.
NATO and Western countries insist that sovereign states like Ukraine are free to decide matters for themselves, including potential progress for membership in the security alliance formed partly as a countermeasure to the Soviet Union in the followingmath of World War II.
Russia claims that the build-up of its forces along the Ukrainian border is its own affair, because it takes place within its own territory. On Sunday, a senior foreign policy official, Yuri Ushakov, described US warnings of an imminent invasion that “hysteria has reached its climax.”