"We don't know whether President (Vladimir) Putin has made the decision to invade. We do know that he is putting in place the capacity to do so in on short order should he so decide," US Secretary of State Antony Blinken said.
"Should Russia follow the path of confrontation, when it comes to Ukraine, we've made clear that we will respond resolutely, including with a range of high impact economic measures that we have refrained from pursuing in the past."
Blinken was speaking in the Latvian capital Riga after conferring with foreign ministers from NATO and Ukraine on how to respond to what Kyiv says is a Russian build-up of more than 90,000 troops near its border.
NATO Secretary General Jens Stoltenberg said the alliance had a range of options should Russia use force against Ukraine, including economic, financial and political measures.
Russia seized the Crimean peninsula from Ukraine in 2014 but denies aggressive intent in the current crisis and says it is responding to threatening behaviour by NATO and Ukraine.
The Kremlin said on Wednesday it feared Ukraine was gearing up to try to recapture pro-Russian separatist regions in the east of the country by force - something Kyiv denies - and accused it of "very dangerous adventurism".
It said Russia could not take any steps to de-escalate because of a large concentration of Ukrainian forces close to the border.
Ukrainian Foreign Minister Dmytro Kuleba said Russia was making dangerous attempts to shift the blame onto Ukraine.
"The European continent may be at a very critical juncture right now. We believe it is necessary to show strength in order to avoid the need to be proving it later," he said.
Blinken said it was part of Russia's playbook "to claim provocations for something they were planning to do all along."
He said Moscow had also stepped up disinformation.
"In recent weeks we’ve also observed a massive spike, more than tenfold, in social media activity pushing anti-Ukrainian propaganda, approaching levels last seen in the lead up to Russia’s invasion of Ukraine in 2014.”
Blinken declined to spell out what sanctions Russia might face, and encouraged both Moscow and Kyiv to return to diplomacy and revive a 2014 peace plan for eastern Ukraine.
Russia has effectively blunted the impact of sanctions imposed over its invasion of Crimea by reducing its borrowings on foreign financial markets and maintaining large currency and gold reserves.
But the West has more potential leverage now if it were to target the newly built Nord Stream 2 pipeline under the Baltic Sea, through which Russia is keen to start pumping gas as soon as it gets the green light from a German regulator.