NATO Secretary General Jens Stoltenberg said on Wednesday that the alliance submitted ideas to Russia for a diplomatic solution to tensions sparked by Moscow’s military buildup in Ukraine, but that the alliance remained “prepared for the worst.”

“We are now reaching out to Russia once more in an attempt to continue a route of conversation and find a political solution,” he added, after the alliance delivered Moscow a formal response to its security requests.

“But, while we hope and strive for a constructive outcome, de-escalation, we are also prepared for the worst,” he warned.

Russian Deputy Foreign Minister Alexander Grushko confirmed to Interfax that Moscow had received NATO’s answer, which had been handed over to Russia’s representative in Brussels.

When asked how long it would take Russia to review the answer, he responded, “We’ll read it.” We’ll look into it. Our partners had studied our manuscript for over a month and a half.”

Stoltenberg said the US-led NATO was ready for a “serious talk” about Russia’s worries, but he rejected a crucial Moscow demand that would effectively end Ukraine’s chances of joining.

“We cannot and will not compromise on the fundamentals that underpin the security of our partnerships, as well as the security of Europe and North America,” he added.

The alliance’s ideas were delivered at the same time as the US’s formal response to the Kremlin.

In December, Moscow surprised the West by posting two draft accords for the US and NATO that would reduce Washington’s influence in Eastern Europe.

The demands came as Moscow amassed 100,000 troops on its border with Ukraine, a move that the West fears might be the precursor to a large-scale invasion.

Stoltenberg outlined a number of areas in which NATO believes it can engage productively with the Kremlin, including improved communications, increased openness surrounding military exercises, and weapons control.

NATO is hopeful that this offer would persuade Moscow to undertake additional discussions with the alliance and de-escalate tensions on the ground.