Russia-Ukraine updates: US to ban Russian carriers from its airspace

President Joe Biden warned Russian President Vladimir Putin in remarks from the White House on Tuesday that if Russia invades Ukraine, the U.S. is prepared to respond decisively and in unison with NATO allies.

"If Russia attacks Ukraine, it would be a war of choice or a war without cause or a reason. I say these things not to provoke but to speak the truth. Because the truth matters. Accountability matters," Biden said, drawing a distinction between the circumstances now and seen in WWII.

Biden said if Russia invades in the days or weeks ahead, the human costs for Ukraine and strategic cost for Russia would be "immense" -- and acknowledged Americans "would also have consequences here at home" if the energy market is hit.

"If Russia attacks Ukraine, it will be met with overwhelming international condemnation," Biden said. "The world will not forget that Russia chose needless death and destruction. Invading Ukraine will prove to be a self-inflicted wound. The United States and our allies and partners will respond decisively. The West is united and galvanized."

He added, "Defending democracy and liberty is never without cost."

President Joe Biden delivered an update from the White House on the status of Ukraine's border and said the U.S. has "not yet verified" that Russian troops have pulled back as the Kremlin has claimed.

"We have not yet verified that the Russian military units are returning to their home bases. Indeed, our analysts remain that they remain very much in a threatening position," Biden said from the White House East Room.

Biden emphasized that the U.S. believes the best way forward is through diplomacy and de-escalation but placed the ball in Putin's court.

"The United States is prepared, no matter what happens. We are ready with diplomacy -- to be engaged in diplomacy with Russia. And our allies and partners to improve stability and security in Europe as a whole. And we are ready to respond decisively to a Russian attack on Ukraine, which is still very much a possibility," he said.

Secretary of State Antony Blinken told his Russian counterpart, Russian Foreign Minister Sergey Lavrov, that the U.S. "looks forward" to receiving Russia's response to U.S. and NATO proposals "within the coming days," according to his spokesperson Ned Price, with both sides noting again in readouts of their call Tuesday there is continued space for diplomatic talks.

Blinken "reiterated our ongoing concerns that Russia has the capacity to launch an invasion of Ukraine at any moment and emphasized the need to see verifiable, credible, meaningful de-escalation," Price said, in a nod to Russia's claims that it has started pulling some troops back, even though U.S. officials say they haven't verified that yet.

On the Russian side, Lavrov "stressed" to Blinken the "need to continue the joint work" on dialogue, including on "the set of issues raised by Russia with emphasis on the principle of indivisible security," according to the Russian Foreign Ministry.

Lavrov and Blinken also discussed "future exchanges at various levels," the Foreign Ministry added. The two are expected to speak again once that Russian response has been transmitted -- and possibly meet in person.

-ABC News' Conor Finnegan and Tanya Stukalova

White House press secretary Jen Psaki emphasized in a tweet that President Joe Biden's remarks at 3:30 p.m. on Russia and Ukraine will "not announce new policy."

"He will speak about the situation on the ground, the steps we have taken, the actions we are prepared to take, what's at stake for the US and the world and how this may impact us at home," she said.

Biden's remarks will air on ABC News and ABC News Live.

Psaki is scheduled to hold a White House press briefing following Biden's remarks and amid skepticism as to whether Russia is choosing de-escalation and withdrawing troops from Ukraine's border as the Kremlin has claimed.