Ukrainian authorities vowed to keep the country's airspace open despite warnings from Western countries that Russian troops carrying out military drills near its borders could launch an invasion at any moment.
The looming threat of the skies over Ukraine closing came with a growing number of Western countries winding down their diplomatic operations in Kyiv and urging their citizens to leave immediately.
It follows a frantic week of urgent but seemingly futile diplomatic efforts to resolve one of the most explosive standoffs between the West and Russia since the Cold War.
U.S. President Joe Biden was due to brief Ukrainian leader Volodymyr Zelenskyy later on Sunday about his hourlong phone conversation with Russian President Vladimir Putin.
The White House reported there had been no breakthrough during Saturday's talks with the Kremlin chief.
French President Emmanuel Macron said he had agreed "to continue dialogue" with Putin in his own telephone conversation that same day.
Western leaders are pushing back against Putin's demands that the U.S.-led NATO alliance withdraw from Eastern Europe and never expand into Ukraine.
But Putin is dismissing calls by Biden and others to pull back Russian forces from Ukraine's frontiers.
Washington has warned that the Russian deployments – estimated at 130,000 soldiers backed by various missiles and tanks – were sufficient to launch a major attack "any day."
German Chancellor Olaf Scholz said on the eve of a crunch trip to Kyiv Monday and Moscow Tuesday that Western allies would "immediately" sanction Russia if it invades.
"In the event of a military aggression against Ukraine that threatens its territorial integrity and sovereignty, that will lead to tough sanctions that we have carefully prepared," Scholz said.
The Dutch carrier KLM on Saturday became the first major airline to indefinitely suspend flights to the former Soviet republic because of the rising risks.
Ukraine's budget airline SkyUp said on Sunday that its flight from Portugal to Kyiv was forced to land in Moldova because the plane's Irish leasing company had revoked permission for it to cross into Ukraine.
SkyUP added that European leasing companies were demanding that Ukrainian airlines return their planes to European Union airspace within 48 hours.
Ukraine's infrastructure ministry responded by holding an emergency meeting aimed at maintaining foreign travel and keeping the country from becoming more isolated in the heat of the crisis.
"The airspace over Ukraine remains open and the state is working on preempting risks for airlines," the ministry said after the meeting.
Industry analysts believe other international airlines may soon also ban flights into Ukraine because of the growing cost to insurers.
The travel industry is still haunted by the memory of Malaysia Airlines flight MH17 being shot down while flying near eastern Ukraine's conflict zone in July 2014.
All 298 passengers aboard the Amsterdam-Kuala Lumpur flight died.
Ukraine's infrastructure ministry acknowledged that "some carriers are facing difficulties linked to fluctuations on the insurance market."
"For its part, the state is prepared to support airlines and provide them with additional financial guarantees in order to support the market," it said.
The worries about air travel come with a growing number of Western governments winding down their missions and advising citizens to get out.
The U.S. State Department on Saturday ordered all nonemergency embassy staff out of Ukraine.
Russia cited fears of "possible provocations from the Kyiv regime" as it also began pulling out some embassy staff.
The diplomatic drawdown has touched the staff of the Organization for Security and Co-operation (OSCE) monitoring mission in Ukraine.
The OSCE has served as the world's eyes and ears for the eight-year conflict across Ukraine's Russian-backed separatist east that has claimed more than 14,000 lives.
But images on social media showed convoys of its white SUVs leaving various parts of the conflict zone as staff moved to comply with their respective governments' travel advisories.
Russian foreign ministry spokesperson Maria Zakharova said Sunday that the mission's partial withdrawal caused "serious concern" in Moscow because the move further ramped up tensions.
The Ukrainian government has been trying to preempt the flood of foreigners leaving the country by calling for calm and criticizing U.S. warnings of possibly imminent war.
Ukrainian President Volodymyr Zelenskyy said on Saturday that "all this information is only provoking panic and not helping us."
Meanwhile, Ukraine criticized British Defense Secretary Ben Wallace's comparison of diplomatic efforts aimed at preventing an invasion by Russia as appeasement, saying now is the wrong time to "offend our partners."
The "worsening situation" in Ukraine caused Wallace, who has warned an attack is "highly likely," to leave a family holiday in Europe early on Sunday, having departed a day earlier.
But his remark that there is a "whiff of Munich in the air," a reference to the agreement that allowed German annexation of the Sudetenland in 1938 but failed to prevent World War II, was not welcomed by Ukraine.
Ambassador to the United Kingdom Vadym Prystaiko warned that the panic is caused by the West sounding the alarm that could be playing into Putin's hands.
"It's not the best time for us to offend our partners in the world, reminding them of this act which actually not bought peace but the opposite, it bought war," the diplomat told BBC Radio 4's Broadcasting House program.
"There's panic everywhere not just in people's minds but in financial markets as well," he added, warning it is "hurting the Ukrainian economy on sort of the same level as people leaving the embassy."
Wallace said in an interview with The Sunday Times that Moscow could "launch an offensive at any time," with an estimated 130,000 Russian troops and heavy firepower amassed along Ukraine's border.
"It may be that he (Putin) just switches off his tanks and we all go home but there is a whiff of Munich in the air from some in the West," he added.
A source close to Wallace explained that his frustrations centered on if Putin strikes "come what may, then all the diplomacy would have been a straw man," rather than being aimed at any European allies.
The defense secretary said he was returning from a planned long weekend abroad in Europe with his wife and children "because we are concerned about the worsening situation in Ukraine."
Wallace arrived back in the U.K. from Moscow in the early hours of Saturday before heading abroad with his family, but it was understood he had already accepted he would be leaving the holiday early rather than having canceled it on arrival in the light of new developments with Russia.
Northern Ireland Secretary Brandon Lewis said an imminent attack is "entirely possible" but insisted Wallace was not criticizing European allies with his Munich remark.
Lewis discussed the optimism of 1938 that diplomacy could prevent European conflict, adding: "It turned out that wasn't the intent or aim of Adolf Hitler at the time."
Ukraine is not a NATO member and allies in the defense alliance have said they would not join fighting in Ukraine but have bolstered forces in neighboring nations and are threatening widespread sanctions.
Though the Kremlin insists it is not planning an invasion, U.S. intelligence suggests Russia could fabricate a "false flag" pretext to attack.