Washington reported Sunday that Moscow has positioned more of its troops closer to Ukraine's borders and that some airlines have canceled or diverted flights from the capital of Kyiv. Ukrainian President Volodymyr Zelenskyy, on the other hand, urged calm amid intensified warnings of a possible Russian invasion within days, saying he had yet to see convincing evidence of that.

United States President Joe Biden spoke for about 50 minutes Sunday with Zelenskyy and renewed promises of what the West says will be tough economic sanctions against Moscow and a NATO buildup in the event of "any further Russian aggression" against Ukraine, the White House said. They agreed to pursue both deterrence and diplomacy in the crisis, it added.

The U.S. updated its estimate for how many Russian forces were now positioned near Ukraine's borders to more than 130,000, up from the 100,000 the U.S. has cited publicly in previous weeks. A U.S. official gave the estimate, speaking on condition of anonymity, to discuss the administration's conclusion.

Zelenskyy's repeated statements playing down the U.S. warnings – while Moscow's forces surround Ukraine on three sides in what the Kremlin insists are military exercises – grew this weekend to his questioning the increasingly strident statements from U.S. officials in recent days that Russia could be planning to invade as soon as midweek.

While Zelenskyy has urged against panic that he fears could undermine Ukraine's economy, he and his civilian and military leaders also are preparing defenses, soliciting and receiving a flow of arms from the U.S. and other NATO members.

Zelenskyy wore military olive drab at a drill with tanks and helicopters near Ukraine's border with Russian-annexed Crimea this weekend. In the nearby city of Kalanchak, some expressed disbelief that Russian President Vladimir Putin would really send the troops poised along Ukraine's borders rolling into the country.

"I don’t believe Russia will attack us," said resident Boris Cherepenko. "I have friends in Sakhalin, in Krasnodar," he said, naming Russian locations. "I don’t believe it."

The U.S. picked up intelligence that Russia is looking at Wednesday as a target date, according to a U.S. official familiar with the findings. The official, who also was not authorized to speak publicly and did so only on condition of anonymity, would not say how definitive the intelligence was.

"We’re not going to give Russia the opportunity to conduct a surprise here, to spring something on Ukraine or the world," Jake Sullivan, the U.S. national security adviser, told CNN on Sunday, about the U.S. warnings.

"We are going to make sure that we are laying out for the world what we see as transparently and plainly as we possibly can," he said.

The U.S. largely has not made public the evidence it says is underlying its most specific warnings on possible Russian planning or timing.

The Russians have deployed missile, air, naval and special operations forces, as well as supplies to sustain an invasion. This week, Russia moved six amphibious assault ships into the Black Sea, augmenting its capability to land on the coast.

Zelenskyy's comments this weekend indicated frustration at the warnings from Washington.

"We understand all the risks, we understand that there are risks," he said in a live broadcast. "If you, or anyone else, has additional information regarding a 100% Russian invasion starting on the 16th, please forward that information to us."

In an hourlong call Saturday with Putin, Biden said an invasion of Ukraine would cause "widespread human suffering" and that the West was committed to diplomacy to end the crisis but "equally prepared for other scenarios," the White House said. It offered no suggestion that the call diminished the threat of an imminent war in Europe.

Reflecting the West's concerns, Dutch airline KLM has canceled flights to Ukraine until further notice, the company said.

Dutch sensitivity to potential danger in Ukrainian airspace is high following the 2014 shooting down of a Malaysian jetliner flying from Amsterdam to Kuala Lumpur as it flew over a part of eastern Ukraine held by Russia-backed rebels. All 298 people aboard died, including 198 Dutch citizens.

The Ukrainian charter airline SkyUp said Sunday its flight from Madeira, Portugal, to Kyiv was diverted to the Moldovan capital of Chisinau after the plane's Irish lessor said it was banning flights in Ukrainian airspace.

Ukrainian presidential spokesperson Serhii Nykyforov told The Associated Press (AP) that Ukraine has not closed its airspace.

But Ukraine’s air traffic safety agency Ukraerorukh issued a statement declaring the airspace over the Black Sea to be a "zone of potential danger" and recommended that planes avoid flying over the sea Feb. 14-19.

The Putin-Biden conversation, following a call between Putin and French President Emmanuel Macron earlier in the day, came at a critical moment for what has become the biggest security crisis between Russia and the West since the Cold War. U.S. officials believe they have mere days to prevent an invasion and enormous bloodshed in Ukraine.

German Chancellor Olaf Scholz will fly to Kyiv on Monday and Moscow on Tuesday to meet with the presidents in those capitals.

While the U.S. and its NATO allies have no plans to send troops to Ukraine to fight Russia, an invasion and resulting punishing sanctions could reverberate far beyond the former Soviet republic, affecting energy supplies, global markets and the power balance in Europe.

Preparing for a worst-case scenario, the United States was pulling most of its staff from the embassy in Kyiv and urged all American citizens to leave Ukraine immediately. Britain joined other European nations in telling its citizens to leave.

Biden has bolstered the U.S. military presence in Europe as reassurance to allies on NATO’s eastern flank. The 3,000 additional soldiers ordered to Poland come on top of 1,700 who are on their way there. The U.S. Army also is shifting 1,000 soldiers from Germany to Romania, which like Poland shares a border with Ukraine.

Russia is demanding that the West keep former Soviet countries out of NATO. It also wants NATO to refrain from deploying weapons near its border and to roll back alliance forces from Eastern Europe – demands flatly rejected by the West.

Russia and Ukraine have been locked in a bitter conflict since 2014, when Ukraine’s Kremlin-friendly leader was driven from office by a popular uprising. Moscow responded by annexing the Crimean Peninsula and then backing a separatist insurgency in eastern Ukraine, where fighting has killed over 14,000 people.

A 2015 peace deal brokered by France and Germany helped halt large-scale battles, but regular skirmishes have continued, and efforts to reach a political settlement have stalled.

"My family has always been prepared, we have all the stuff gathered for like a couple of years now. Honestly, I’m not afraid because the war wouldn’t start like in a week," 21-year-old Yuliia Zaets said at a pro-government rally on Saturday.