"We are prepared to discuss them. That said, there are some things in this document that the Russians know will be unacceptable," the official told reporters on condition of anonymity.

The official said the United States expected to respond to the Russians "sometime next week with a more concrete proposal" on talks after consulting with European allies, which the United States insists need to be part of any discussions.

Russia took the unusual step of presenting draft agreements on security amid mounting concerns over its amassing of tens of thousands of troops by the Ukrainian border.

The US official renewed a warning that the West would not tolerate an invasion, which would come after seven years of a Russian-supported insurgency in eastern Ukraine.

"If there is any further aggression against Ukraine, that will have massive, massive consequences and will carry a high price," the official said.

She said that any retaliation would consist "largely" of economic measures.

But she warned: "We are prepared to consider a number of things that we had not considered in the past, and the results will be very profound on the Russian Federation."