The US ejected Russia's demand to bar Ukraine from NATO and said it believed Moscow was ready to invade.
"Putting things in writing is ... a good way to make sure we're as precise as possible, and the Russians understand our positions, our ideas, as clearly as possible. Right now, the document is with them and the ball is in their court," he said.
In Paris, diplomats from Russia, Ukraine, France and Germany held more than eight hours of talks on ending a separatist conflict in eastern Ukraine, part of the wider crisis between Moscow and Kyiv that risks becoming a full-scale war. Russia meanwhile staged new military drills on land and on the Black Sea and moved more paratroopers and fighter jets to Belarus, north of Ukraine, for what it describes as joint exercises there next month.