Russia could invade Ukraine 'any day now', a top White House security advisor has warned.
Moscow has assembled almost three-quarters of the necessary military firepower required to launch an invasion in Ukraine this month, US intelligence reports say.
It is said that there are an estimated 100,000 troops near Ukraine, and the Kremlin has said it will station troops wherever it needs to in Russia.
Moscow, which seized Crimea from Ukraine in 2014, has denied planning an invasion.
White House national security advisor Jake Sullivan offered a stark warning over what will happen if there is an invasion.
Mr Sullivan said on Sunday: “If war breaks out, it will come at an enormous human cost to Ukraine, but we believe that based on our preparations and our response, it will come at a strategic cost to Russia as well.”
It comes after the White House briefed members of congress that a full Russian invasion could lead to the quick capture of Kyiv and could result in as many as 50,000 casualties.
Mr Sullivan did not address the reports as he spoke to Fox News Sunday.
He added: "We are in the window. Any day now, Russia could take military action against Ukraine, or it could be a couple of weeks from now, or Russia could choose to take the diplomatic path instead."
His worrying comments come after two US officials warned Russia was about 70 per cent ready for a full-scale invasion of Ukraine.
Yesterday, hundreds of US airborne infantry troops landed in Poland.
US President Joe Biden is planning on deploying 3,000 soldiers to Germany, Poland and Romania, increasing its presence in the region.
Foreign Secretary Liz Truss has accused Russia of trying to “subvert and threaten Ukraine”.
Ms Truss, who is also expected to visit Moscow next week, shared an article on Twitter about the claims that Russia could “seize Kyiv in days” and cause “up to 50,000 civilian deaths”.
She wrote: “Russia’s actions show their claims to have no plans to invade are false.”
Russian officials have dismissed the warnings.
Russia’s deputy ambassador to the United Nations Dmitry Polyanskiy said: "Madness and scaremongering continues.
“What if we would say that US could seize London in a week and cause 300K civilian deaths? All this based on our intelligence sources that we won’t disclose.
“Would it feel right for Americans and Brits? It’s as wrong for Russians and Ukrainians.”
Ukraine’s foreign minister Dmytro Kuleba has also weighed in and wrote on Twitter: "Different capitals have different scenarios, but Ukraine is ready for any development.
“Today, Ukraine has a strong army, unprecedented international support and the faith of Ukrainians in their country. It is the enemy who should fear us.”
Western allies have threatened extensive economic sanctions against Russia.
It comes as Germany has also been accused of not pulling its weight by allies to help simmer the escalating tension in the region.
Nato members have deployed battlegroups, send tactical supplies and offer more vocal support.
However, Germany appears to have refused to offer tangible support.
German Chancellor Olaf Scholz will travel to the White House next week to assure the US and other Nato that it stands alongside its allies in opposing any Russian aggression against Ukraine.
Sen. Richard Blumenthal, a Democrat and member of the Armed Services Committee, recently told an audience of Ukrainian Americans in his state, Connecticut: "The Germans are right now missing in action. They are doing far less than they need to do."
