America and its allies hit back at Russia’s attempted annexation of four Ukrainian regions on Friday, slapping sanctions on more than 1,000 people and companies, including arms supply networks, as Moscow and the West escalated an already heated conflict fraught with potential nuclear implications.

President Biden warned Vladimir Putin he can’t “get away with” seizing Ukrainian land. American officials said they would support any effort by Ukraine to retake the annexed territories by force, setting the stage for further hostilities. 

President Zelensky announced that his country would make an “accelerated” bid to join NATO, a plan not endorsed by the U.S. or other allies and that could add fuel to the fire.

Mr. Putin’s land-grab and Mr. Zelensky’s renewed push for NATO membership sent the two leaders speeding faster on a collision course that is cranking up fears of a full-blown conflict between Russia and the West.

In a hastily convened press conference late on Friday, the NATO secretary general, Jens Stoltenberg, made clear the military alliance’s support for Ukraine as the war enters a critical inflection point, but appeared to draw a line at Ukraine’s joining the bloc. “NATO is not party to the conflict,” Mr. Stoltenberg told reporters at Brussels. “We support Ukraine, but that doesn’t make us party to the conflict,” he emphasized, adding, “We support a sovereign nation in their sovereign right to self-defense. 

With respect to Moscow’s recent escalatory rhetoric about nuclear weapons, Mr. Stoltenberg said that as of today, “We haven’t seen any changes in Russia’s nuclear posture, but we are vigilant. And we have conveyed very clearly to Russia that there will be severe consequences if they use nuclear forces against Ukraine.”

Mr. Stoltenberg also tried to tamp down tensions over the risk of the war spreading beyond Ukraine. “The morning of the invasion,” he said, “we activated our defense plans from the Baltic Sea to the Black Sea and started deployment of additional troops to the eastern part of the alliance, to remove any room for miscalculation or misunderstanding in Moscow about our willingness and readiness to protect every inch of NATO territory.”

Mr. Putin has repeatedly made clear that any prospect of Ukraine joining the military alliance is one of his red lines and cited it as a justification for his invasion, now in its eighth month, in the biggest land war in Europe since World War II.

Earlier in the day at Moscow, Mr. Putin vowed to protect newly annexed regions of Ukraine by “all available means,” a renewed nuclear-backed threat he made at a Kremlin signing ceremony where he also railed furiously against the West, accusing the United States and its allies of seeking Russia’s destruction.

Mr. Zelensky then held his own signing ceremony in Kyiv, releasing video of him putting pen to papers he said were a formal NATO membership request.

In his speech, Mr. Putin urged Ukraine to sit down for peace talks but immediately insisted he won’t discuss handing back occupied regions. Mr. Zelensky said there’d be no negotiations with Mr. Putin.

At his signing ceremony in the Kremlin’s ornate St. George’s Hall, Mr. Putin accused the West of fueling the hostilities as part of what he called a plan to turn Russia into a “colony” and a “crowd of soulless slaves.” The hardening of his position, in the conflict that has killed and wounded tens of thousands of people, further raised tensions already at levels unseen since the Cold War.

Western countries responded with an avalanche of condemnation, more punishment for Russia, and aid for Ukraine. The U.S. announced sanctions for more than 1,000 people and firms connected to Russia’s invasion, including its Central Bank governor.

Of Mr. Putin’s annexation of the Donetsk, Luhansk, Kherson, and Zaporizhzhia regions, Mr. Biden said: “Make no mistake: These actions have no legitimacy.”

At Brussels, Mr. Stoltenberg called it “the largest attempted annexation of European territory by force since the Second World War” and said that the war is at “a pivotal moment.”