John Hamilton | U.S. Army via AP

Moscow signaled to the West that it's ready for a nuclear confrontation after Ukraine was given permission to attack Russian territory — and appeared to quickly act on that greenlight — using U.S.-made long-range missiles.

Kyiv appeared to waste little time after reportedly being given the go-ahead by Washington on Sunday to use U.S.-made ATACMS missiles against specific targets. Ukrainian news outlets reported early Tuesday that the missiles had been used to attack a Russian military facility in the Bryansk border region.

Russia then confirmed the attack, with the Ministry of Defense stating that Ukrainian forces had "struck a facility in [the] Bryansk region" using six ballistic, American-made ATACMS missiles. The ministry claimed air defense missile systems had shot down five of the missiles, and damaged another.

"Its fragments fell on the technical territory of a military facility in the Bryansk region, causing a fire that was quickly extinguished. There were no casualties or damage," the ministry said.

CNBC was unable to independently verify the reports and Ukraine's leadership has not yet commented on the attack. The Kyiv Post news outlet cited a national security official as confirming the strike in Bryansk had been carried out, although he did not indicate which weapons had been used.

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Russia's foreign minister, Sergey Lavrov, responded to the attack, accusing the West of wanting to escalate the conflict.

"The fact that ATACMS were used repeatedly tonight in the Bryansk region is, of course, a signal that they [in the West] want escalation. And without the Americans, it is impossible to use these high-tech missiles," Lavrov said at a news conference at the G20 summit, according to comments reported by Tass and translated by Google.

The Kremlin has repeatedly warned the West against allowing Ukraine to use its long-range weapons to attack Russia directly. Moscow upped the ante Tuesday as Russian President Vladimir Putin signed a decree approving its updated nuclear doctrine, shifting the parameters on when Russia can use nuclear weapons.

The updated document now states that any aggression against Russia by a non-nuclear state, if it's supported by a nuclear power, will be considered as a joint attack.

The doctrine also stated that Russia may use nuclear weapons in the event of a critical threat to its sovereignty and territorial integrity (and that of its ally, Belarus) and that the launch of ballistic missiles against Russia would be seen among the conditions that could warrant a response using nuclear weapons.

The Kremlin's press secretary Dmitry Peskov was asked on Tuesday whether Russia would consider the use of American non-nuclear missiles by the Ukrainian military as an attack by a non-nuclear state with the support of a nuclear state.

​"You will be able to read the paragraphs yourself, but in general it also states that the Russian Federation reserves the right to use nuclear weapons in the event of aggression with the use of conventional weapons against it or the Republic of Belarus, which creates a critical threat to sovereignty or territorial integrity," Peskov told reporters.

"Aggression against the Russian Federation by any non-nuclear state with the participation or support of a nuclear state is considered a joint attack."

Is Russia bluffing?

Heightened tensions between Russia and Ukraine, and its allies in the West, come as Russian forces are seen to be making considerable gains in eastern Ukraine, looking to seize as much territory as possible before President-elect Donald Trump takes office in January.

It's widely expected that Trump will push Moscow and Kyiv into peace talks that will be favorable to Russia, forcing Ukraine to concede occupied land to its neighbor as a price for peace.

Trump has already signaled that U.S. military aid for Ukraine will end when he returns to office and some Republican officials are reportedly unhappy at the Biden administration for giving Ukraine permission to use ATACMS.

There are also doubts that the limited number of ATACMS Ukraine is believed to have been given by the U.S. will be enough to change the dial in the war, which reached its 1,000-day mark on Tuesday.

"The impact may be more political, albeit with a narrowing window of opportunity," Matthew Savill, the military sciences director at the Royal United Services Institute defense think tank, said in comments Monday.

"The Ukrainians need to convince the incoming U.S. administration that they are still worth backing — in President Trump's transactional view, a 'good investment'. And they will want to convince him to link his and U.S. credibility to a 'winning' outcome, not a major compromise that sees the U.S. 'lose'. However, the signs are not positive on this front, with criticism overnight from amongst the Trump campaign of what they may see as a Biden attempt to tie Trump's hands," he said.

"Moreover, it is unlikely that the Ukrainians can have a major impact in such a short period of time (under two months) until the next administration is formally in place, and with an uncertain but probably small stockpile of ATACMS," he said.

Vyacheslav Prokofyev | Via Reuters

In the meantime, the immediate question for defense and geopolitical analysts is how Russia assesses Ukraine's attack against its territory using U.S. weapons, and whether it will react within the confines of its updated nuclear doctrine. Throughout the war, Russia has warned it is prepared to use its nuclear weapons if under attack, but analysts say Moscow would be hesitant to go head-to-head against the combined might of Western military alliance NATO, and the nuclear powers within the bloc.

Global markets are taking the threat seriously, at least, with stocks declining and investors fleeing to safe haven assets on Tuesday, although some close followers of Russia's leadership believe threats issued by the Kremlin are just another instance of saber-rattling.

"Putin is bluffing again," Timothy Ash, emerging markets strategist at BlueBay Asset Management, said in emailed comments Tuesday.

"Putin's bluff was and has ... constantly been called — Putin is terrified of getting into a conventional war with NATO which he would likely lose in weeks," he said.

Ash said Russia was now more likely to respond with "asymmetric warfare," with the attack this week on an undersea cable between Germany and Scandinavia fitting this narrative. Russia has not commented on the attack and has denied previous assaults on subsea energy infrastructure in Europe.

"We should expect more of the same, but particularly targeted at Europe, not the U.S.," Ash said, adding: "He [Putin] exploits weakness. He now sees that with the new Trump administration in the U.S. Trump should not be pulled in by Putin's games. Trump should call Putin's bluff."

Bomb shelters

Whether the Kremlin is bluffing or not, there are signs that Russia's leadership takes the threat of a nuclear confrontation seriously — or at least wants to show the Russian public that it is ready for such an eventuality. Russia's state-run Civil Defense and Emergencies research institute, a part of Russia's Emergencies Ministry, said Monday mobile bomb shelters it had developed had gone into mass production for the first time in history.

The shelters, called "KUB-M" units, look like shipping containers and provide 48-hour protection for people from various threats, including "natural disasters and man-made accidents," including the "air shock wave and light radiation of a nuclear explosion," penetrating radiation and radioactive contamination of an area, the institute said Monday.

The units can also protect from hazardous chemicals, fires and conventional weapons, the research institute said, adding that the key advantage of its KUB-M units was their mobility. Each container can shelter 54 people, it added, with additional capacity possible if more modules were fitted.

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The institute did not link its announcement to developments regarding the ATACMS missiles, or the approval of Russia's updated nuclear doctrine.

The timing of the announcement is not seen to be coincidental, however, acting as another warning to the West that Russia is serious when it warns of a possible nuclear confrontation, and is actively preparing for the possibility, despite repeatedly stating that it does not want a nuclear war with its adversaries.