(CNN) — President Joe Biden warned Russian President Vladimir Putin that the US and its allies will respond “decisively and impose swift and severe consequences” on Russia should Putin decide to invade Ukraine.

In a roughly hour-long phone call, the White House said Biden made clear to Putin what he would be risking with an invasion. A senior administration official told reporters following the call that the discussion was substantive but the US fears Russia may still launch a military attack anyway.

“President Biden was clear that, if Russia undertakes a further invasion of Ukraine, the United States together with our Allies and partners will respond decisively and impose swift and severe costs on Russia. President Biden reiterated that a further Russian invasion of Ukraine would produce widespread human suffering and diminish Russia’s standing,” the White House said in a statement, adding Biden “was clear with President Putin that while the United States remains prepared to engage in diplomacy, in full coordination with our Allies and partners, we are equally prepared for other scenarios.”

The call between the two leaders comes hours after the US moved some of its forces out of Ukraine and ordered the evacuation of most of its embassy staff on Saturday as fears mount that a Russian invasion of the country could take place in the next few days. The moves were yet another sign that the US fears Putin could order an invasion at any time, just one day after Biden’s national security adviser warned Americans in Ukraine to leave and that military action could begin with an aerial bombardment that could kill civilians.

A senior administration official told reporters Saturday afternoon that the call between the two presidents was “professional and substantive,” but “there was no fundamental change in the dynamic that has been unfolding now for several weeks.”

**President Joe Biden and Russian President Vladimir Putin spoke on Saturday, as tension builds over the Russian threat to Ukraine.**

“The two Presidents agreed that our teams will stay engaged in the days ahead,” the official told reporters after the call. “Russia may decide to proceed with military action anyway. Indeed, that is a distinct possibility.”

The official said that Biden reiterated the US’ ideas on how to enhance European security while also addressing some of Russia’s security concerns, but noted that it “remains unclear whether Russia is interested in pursuing its goals diplomatically.”

Asked whether Russia has made a decision to invade, the official said, “I think the honest answer to that question is we don’t have full visibility into President Putin’s decision making.”

“But you know, we are not basing our assessment of this on what the Russians say publicly,” the official continued. “We are basing his assessment on what we are seeing on the ground … which is a continued Russian buildup on the border with Ukraine, and no meaningful evidence of de-escalation, or really of any interest in de-escalation.”

Russian presidential aide Yury Ushakov described the call Saturday as “balanced and businesslike,” but said the US and NATO had failed to address Russia’s main security concerns.

Ushakov said the conversation “took place in an atmosphere of hysteria about the supposedly imminent Russian invasion of Ukraine by American officials, everyone knows this.”

Ushakov added: “The pressure around the topic of the invasion was carried out in a coordinated manner and the hysteria has reached its climax.”

According to Ushakov, Biden told Putin he was “committed to the diplomatic path and had laid out a range of considerations that he sees as addressing many of Russia’s concerns.” Putin said the US and its allies had been “pumping up” Ukraine with new weaponry and encouraging provocations by Ukrainian forces in the Donbas region and in Crimea, Ushakov said.

The Russian Ministry of Foreign Affairs had on Friday accused Western countries and the press of spreading a “large-scale disinformation campaign” about an allegedly impending Russian invasion of Ukraine “in order to divert attention from their own aggressive actions.”

“At the end of 2021 and the beginning of 2022, the global information space faced a media campaign unprecedented in its scale and sophistication, the purpose of which is to convince the world community that the Russian Federation is preparing an invasion of the territory of Ukraine,” the Ministry said in a statement published on its website.

All eyes on Ukraine

The US has estimated that Russia has more than 100,000 troops near the Ukraine border, with thousands added just this week, according to an administration official. National security adviser Jake Sullivan told reporters on Friday that a Russian assault on Ukraine could begin soon, beginning with aerial bombings and missile attacks. He advised all Americans in Ukraine to depart the country for their own safety as quickly as possible.

“We obviously cannot predict the future, we don’t know exactly what is going to happen. But the risk is now high enough and the threat is now immediate enough that this is what prudence demands,” he said.

In Ukraine’s capital, Kyiv, Mayor Vitali Klitschko outlined preparations the city is taking to safeguard “critical and social infrastructure facilities” in “the event of a possible emergency.”

In the statement issued on Telegram ahead of the weekend, Klitschko said, “Our efforts are aimed at preventing or overcoming both possible provocations and withstanding a military attack.”

The senior administration official on Saturday said that US will continue, and even increase, its support for Ukraine to help it defend itself should Russia continue to escalate its aggression.

“As to our plans going forward, I think President Biden and other officials have been clear, that should Russia continue down the path to escalation, the United States will continue to increase our support to Ukraine, to enable it to defend itself, and you know, that approach has not changed,” the official said.

The call on Saturday was the latest attempt at diplomacy between the two leaders aimed at de-escalating the situation. Putin and Biden had last spoke on the phone late last year. Prior to that, on December 7, they had negotiations via video-conference. The first face-to-face meeting between Putin and Biden as heads of state took place in Geneva in June 2021.

The Russian President has also engaged a series of Western leaders in talks that have so far appeared fruitless in defusing the situation.

The attempts at diplomacy with the Russians would continue, the senior administration official said.

“We remain committed to keeping the prospect of de-escalation through diplomacy alive,” the official said. “But we’re also clear eyed about the prospects of that, given the readily apparent steps Russia is taking on the ground in plain sight, right before our eyes. The stakes of this are too high not to give Russia every chance to avoid an action that we believe would be catastrophic. So as always, we continue along two paths.”

Biden, who is spending the weekend at Camp David, took part in a virtual meeting from the White House on Friday with leaders from Canada, the United Kingdom, France, Germany, Italy, Poland, Romania, NATO, the European Commission and the European Council to discuss escalating tensions.

Along with the US, several other countries are calling for their citizens to leave Ukraine immediately as well. On Friday, a British diplomat told CNN that the British Embassy in Kyiv is “temporarily removing all nonessential staff and dependents” and “a core team will remain to continue with essential duties.”