The US has rejected Russia’s demand to bar Ukraine from Nato, amid warnings Russia might invade its neighbour.
Secretary of State Antony Blinken was giving Russia a formal response to its demands to resolve the Ukraine crisis.
Mr Blinken gave no concessions but said that he was offering Russia “a serious diplomatic path forward, should Russia choose it”.
A Russian minister said his country would study Mr Blinken’s response, delivered in coordination with Nato.
Russia had issued a written list of its concerns about the expansion of the Nato military alliance and related security issues.
Among them was a demand for Nato to rule out the possibility of Ukraine and others ever joining the alliance.
In recent weeks, Russia has been amassing large numbers of troops on the Ukraine border – something which Western countries have seen as preparation for a possible invasion. Russia denies this.
Mr Blinken said the US response made its “core principles” clear, including Ukraine’s sovereignty and its right to choose to be part of security alliances such as Nato.
“There should be no doubt about our seriousness of purpose when it comes to diplomacy, and we’re acting with equal focus and force to bolster Ukraine’s defences and prepare a swift united response to further Russian aggression,” he said.