US President Joe Biden and Russia’s Vladimir Putin will speak via video call on Tuesday, the White House says, amid mounting tensions over Ukraine.
It comes after Secretary of State Antony Blinken said the US had evidence that Russia had made plans for a “large scale” attack on Ukraine.
But he added it was unclear if Mr Putin had made a final decision to invade.
Russia has denied any such intention, and accused Ukraine of executing its own troop build-up.
In a statement released on Saturday evening, White House Press Secretary Jen Psaki said that Mr Biden will “underscore US concerns with Russian military activities on the border with Ukraine and reaffirm the United States’ support for the sovereignty and territorial integrity of Ukraine” during his call with the Russian president.
Ukraine says Russia has deployed armoured vehicles, electronic warfare systems and 94,000 troops along their shared border.
It is the largest massing of Russian forces on its borders since it annexed Crimea from Ukraine in 2014.
Citing intelligence reports, Ukraine’s Defence Minister Oleksii Reznikov said Moscow could be planning a military offensive at the end of January.
The Russian troop movement has strained already tense relations between Russia and the US.
The US and its European allies have discussed imposing sanctions on Russia if it takes aggressive action.
While Ukraine is not a Nato member, it has close ties with the bloc and has received Western weapons including US Javelin anti-tank missiles.
Russian officials have denied any plans for an invasion, and say the border troops are there for military exercises.
Moscow has accused Nato of engaging in provocative behaviour by holding drills in the Black Sea, off Crimea. Russia’s foreign ministry also said Ukraine has itself sent 125,000 troops to their shared border. Kyiv declined to comment on the claim.
Gen Sir Nick Carter told the BBC that he “distinctly hoped” there would not be a war with Russia, but added that Nato would have to be ready for that eventuality.
Tensions between Russia and Ukraine are nothing new. In 2014 Russia annexed Ukraine’s Crimean Peninsula and soon after started to back a separatist insurgency in Ukraine’s east.
More recently, Ukraine’s President Volodymyr Zelensky has imposed sanctions on a powerful friend of President Putin and banned broadcasts by three pro-Russian TV stations.
Presidents Biden and Putin held their only face-to-face talks in Geneva in June. Reuters reports that their last phone call was on 9 July.