Speaking on CNN, Secretary Blinken said "everything we are seeing suggests that this is dead serious" and that "we are on the brink of an invasion".
"Until the tanks are actually rolling, and the planes are flying, we will use every opportunity and every minute we have to see if diplomacy can still dissuade President Putin from carrying this forward," he added.
But Ukraine's Defence Minister Oleksii Reznikov said an attack "tomorrow or the day after tomorrow" was unlikely as no Russian "strike groups" had yet formed near the border.
His comments came as unverified reports in the US media suggested Washington believed an attack could be launched imminently.
CBS News reported the US had intelligence that Russian commanders on the ground had received orders to proceed with an invasion and were now making specific battle plans on how to attack.
The report said an invasion would begin with a cyber-assault followed by a campaign of missile and airstrikes, before ground units attempted to take the capital Kyiv.
And
an unnamed intelligence official told CNN close to 75% of Russia's conventional forces were now poised at the Ukrainian border. The concentration of Russian forces within striking distance of Ukraine was highly unusual, the source said.
Again Ukraine's Defence Minister Oleksii Reznikov said an attack "tomorrow or the day after tomorrow" was unlikely as no Russian "strike groups" had yet formed near the border.
In other developments:
The meeting, proposed by France, will only take place if Russia does not invade Ukraine, the US says.
www.bbc.com