The US response to Russian demands amid a crisis regarding Ukraine bucked a key request from the Kremlin, Secretary of State Antony Blinken acknowledged Wednesday.
The document, hand-delivered by US Ambassador John Sullivan in Moscow, reiterates previous public statements in which NATO maintained it would not cave to Russia's demands that Ukraine be barred from membership, Blinken told reporters.
"We will uphold the principle of NATO's open door," the top diplomat said, maintaining it is "a commitment that we're bound to."
"NATO's door is open, remains open, and that is our commitment," he added.
The documents come amid a stark crisis between the West and Russia concerning Ukraine, and Blinken said it would not be made publicly available to ensure it has "the best chance to succeed."
Blinken said the US response includes proposals to address "other areas where we see potential for progress," including arms control, missiles in Europe and a follow-on agreement to a 2011 nuclear arms pact known as New START that places limits on the number of nuclear arms in the US and Russian arsenals.
Russia has amassed more than 120,000 troops on its border with the former Soviet republic alongside a mass deployment of armored vehicles and artillery.
A similar buildup has been seen in Belarus, just north of Ukraine. In both cases, Russia denies preparing for an invasion and maintains its forces are there for regular exercises.
The US and its European allies have warned Russia that it will pay dearly for any military offensive into Ukraine, including through the imposition of what they say are unprecedented economic sanctions.
NATO began to increase its presence in eastern Europe following Russia's illegal annexation of Ukraine's Crimean Peninsula in 2014. That year also saw the Kremlin begin its support for pro-Russian separatists in eastern Ukraine, a policy it has continued for the past eight years.
The US this week sent three shipments of military aid to Ukraine that included additional Javelin and other anti-tank armaments, as well as 283 tons of ammunition and non-lethal equipment, which Blinken said are "essential" for Ukraine's armed forces.
"More deliveries are expected in the days to come," he added.
