A nuclear test in Siberia; cutting undersea internet and telephone cables; supplying rockets to Middle East proxies to sink American ships. These are some options weighed Friday by Russian analysts in response to President Putin’s declaration Thursday that Russia will be at war with NATO if America and Britain allow Ukraine to use  Western-supplied missiles to hit targets deep inside Russia.

President Biden on Friday met at Washington with the visiting British premier, Keir Starmer. No decision on the missiles was expected to be announced, the White House national security spokesman, John Kirby, told reporters. A green light might come in 10 days, after Mr. Biden meets President Zelensky on the sidelines of the UN General Assembly at New York.

Russian state TV repeatedly aired Mr. Putin saying at St. Petersburg on Thursday that the use of Western missiles would change “the very essence” of the Russia-Ukraine war. “It is a question of deciding whether or not NATO countries are directly involved in a military conflict,” he said. “If this decision is taken, it will mean nothing less than the direct involvement of NATO countries, the United States, and European countries in the war in Ukraine.”

The Kremlin spokesman, Dmitry Peskov, during a briefing with reporters Friday, said: “The statement made by President Putin yesterday is very important. It is extremely clear, unambiguous and does not allow for double readings. We have no doubt that this statement reached those it was intended for.” Mr. Putin had complained that Ukraine would use NATO missileers and GPS positioning for targetry.

By clearly marking out such a public position, it will be difficult for the Russian president to ignore the crossing of yet another of his red lines. Taking a cue from the Kremlin, the Federal Security Service announced that six British diplomats who left Moscow last month had actually been expelled for spying. Timed to pressure the British leader while he’s visiting Washington, the announcement was followed by Russian TV airing photos of the diplomats and a grainy video of one meeting a Russian in a public place.

“It is not just a question of formality and non-compliance with declared activities, but of subversive actions aimed at damaging our people,” the Russian foreign affairs ministry spokeswoman, Maria Zakharova, said Friday on Telegram. Reflecting tense East-West relations, Russian news organization RBC reported last year that Western countries and Japan expelled 670 Russian diplomats between the beginning of 2022 and October 2023. In response, Moscow expelled 346 diplomats. According to RBC, this number was more than in the previous 20 years combined.

From New York on Friday, Russia’s ambassador to the United Nations, Vassily Nebenzia, warned the Security Council that NATO runs the risk of “being a direct party to hostilities against a nuclear power.” He said: “You shouldn’t forget about this — and think about the consequences.” Russia, the world’s largest nuclear power, is revising its nuclear doctrine, the circumstances under which Moscow would use nuclear weapons.

A former Kremlin adviser, Sergei Markov, listed Friday on his Telegram channel the steps that Russia could take against Britain: close the British embassy at Moscow, strike British drones and warplanes over the Black Sea, and fire missiles at F-16 warplanes that carry the Storm Shadows from their NATO bases in Romania and Poland. 

The Franco-British Storm Shadow/SCALP missiles can fly 340 miles at Mach 1. American permission is needed because key components are made in America. The version of American ATACMS missiles delivered to Ukraine can fly 190 miles at Mach 3.

Western analysts say Washington and London should not worry too much about Mr. Putin’s latest line in the sand. For the last year, Ukraine has used ATACMS and Storm Shadows to blast targets in Russia-controlled Crimea. For these attacks, Ukrainian missileers, trained by NATO counterparts, have relied on Western GPS for targetry. Washington and London have authorized these attacks because they consider Crimea part of Ukraine. Mr. Putin considers Crimea part of Russia.

All summer, as debate raged over the possibility of Western-supplied missiles hitting targets inside Russia, Russia’s air force quietly moved almost all its bombers outside of missile range, the American defense secretary, Lloyd Austin, said last week. The Washington-based Institute for the Study of War reports that Russia still has 200 major targets — largely ammunition dumps and logistical hubs — within range of the American and British missiles.

Advocates of using Western-supplied cruise missiles to hit military targets in a 200-mile-wide band of western Russia charge that the Kremlin’s arguments are based on a double standard: Russia can use Iranian and North Korean missiles to hit Ukraine, but Ukraine is not allowed to use American and British missiles to hit Russia.

Indeed, a former Russian defense minister, Sergei Shoigu, met Friday at Pyongyang with the North Korean leader, Kim Jong-un. The meeting follows a state visit by Mr. Putin to Pyongyang in June. Over the last year, North Korea shipped more than 16,500 containers of munitions and war matériel to Russia, the deputy assistant secretary of state, Robert Koepcke, said last week. During that time, Russia launched more than 65 North Korean missiles at targets in Ukraine.

For some Western observers of Russia, “escalation management” is the new appeasement.

“The Russian dictator now routinely threatens Western leaders with nuclear apocalypse if they dare to disrupt his methodical destruction of Europe’s largest nation,” an Atlantic Council editor, Peter Dickinson, writes from Kyiv in a new essay titled “Escalation management is the appeasement of the 21st century.”

“International response to Russia’s invasion has been hampered at every turn by delays and indecisiveness, with Kyiv’s partners denying the country vital weapons and imposing absurd restrictions on Ukraine’s ability to defend itself,” Mr. Dickinson, a Briton who edits the council’s Ukraine Alert website, writes. “As a result, the Ukrainian military currently finds itself forced to fight an existential war with one hand tied behind its back.”