VIENNA (NYTIMES) - No one expected much progress from this past week's diplomatic marathon to defuse the security crisis that Russia has ignited in Eastern Europe by surrounding Ukraine on three sides with 100,000 troops and then, by the White House's accounting, sending in saboteurs to create a pretext for invasion.

But as the Biden administration and North Atlantic Treaty Organisation (Nato) conduct tabletop simulations about how the next few months could unfold, they are increasingly wary of another set of options for President Vladimir Putin, steps that are more far-reaching than simply rolling his troops and armour over Ukraine's border.

Mr Putin wants to extend Russia's sphere of influence to Eastern Europe and secure written commitments that Nato will never again enlarge.

If he is frustrated in reaching that goal, some of his aides suggested on the sidelines of the negotiations last week, then he would pursue Russia's security interests with results that would be felt acutely in Europe and the United States.

There were hints, never quite spelled out, that nuclear weapons could be shifted to places - perhaps not far from the US coastline - that would reduce warning times after a launch to as short as five minutes, potentially igniting a confrontation with echoes of the 1962 Cuban Missile Crisis.

"A hypothetical Russian invasion of Ukraine would not undermine the security of the United States," said Mr Dmitry Suslov, an analyst in Moscow who gave a closed-door presentation on the stand-off to Russian lawmakers last month. "The overall logic of Russian actions is that it is the US and Nato that must pay a high price."

And as Ukrainians were reminded anew on Friday (Jan 14) as the websites of the country's ministries were defaced in a somewhat amateurish attack, Russia's army of hackers can wreak havoc in Ukraine, and also in power grids from Munich to Michigan.

It could all be bluster, part of a Kremlin campaign of intimidation and a way of reminding President Joe Biden that while he wants to focus the US' attention on competing and dealing with China, Mr Putin is still capable of causing enormous disruption.

The Russian leader telegraphed that approach himself by warning repeatedly in the past year that if the West crossed the ever-shifting "red line" that, in Mr Putin's mind, threatens Russia's security, he would order an unexpected response.

"Russia's response will be asymmetrical, fast and tough," Mr Putin said last April, referring to the kinds of unconventional military action that Russia could take if adversaries threatened "our fundamental security interests".

The current crisis was touched off by the Kremlin's release of a series of demands that, if the US and its allies agreed, would effectively restore Russia's sphere of influence close to Soviet-era lines, before Nato expanded into Eastern Europe.

It has also demanded that all US nuclear weapons be withdrawn from Europe, saying it felt threatened by their presence - though the types and locations of those weapons have not changed in years. And it wants a stop to all Western troop rotations through former Warsaw Pact states that have since joined Nato.

It has reinforced those demands, which the US calls "non-starters", with a troop build-up near Ukraine and repeated warnings that it was prepared to use unspecified "military-technical means" to defend what it considers its legitimate security interests.

In response, the Biden administration has issued warnings of financial and technological sanctions if the Kremlin should follow through with its threats, particularly in regard to Ukraine.

American officials say that for all the talk about moving nuclear weapons or using asymmetrical attacks, so far, the US has seen little evidence.

At a White House briefing on Thursday, Mr Jake Sullivan, Mr Biden's national security adviser, declined to be drawn into the question of what kind of Russian action would trigger a US response - whether, for example, the US would respond to a cyber attack the way it would an incursion into Ukrainian territory.

"The United States and our allies are prepared for any contingency, any eventuality," he said. "We're prepared to keep moving forward down the diplomatic path in good faith, and we're prepared to respond to fresh acts. And beyond that, all we can do is get ready. And we are ready."

Of course, the most obvious scenario, given the scale of troop movements on the ground, is a Russian invasion of Ukraine - maybe not to take over the entire country, but to send troops into the breakaway regions around the cities of Donetsk and Luhansk, or to roll all the way to the Dnieper River.

At the Pentagon, "five or six different options" for the extent of a Russian invasion are being examined, one senior official reported.

Researchers tracking social media footage have spotted numerous signs of additional Russian military equipment being shipped westwards by train from Siberia.

In Russia, state television has been filled with commentators' warnings that Ukraine could soon attack Russian-backed separatists in eastern Ukraine - fitting with Washington's allegation on Friday that Russian operatives, with specialties in explosives and urban warfare, have infiltrated Ukraine and might be planning to stage a provocation to justify an invasion. Russia denied the allegation.

In Geneva, Russian diplomats insisted there were no plans to invade Ukraine. But there were hints of other steps.

In one little-noticed remark, a senior Russian diplomat said Moscow was prepared to place unspecified weapons systems in unspecified places. That merged with US intelligence assessments that Russia could be considering new nuclear deployments, perhaps tactical nuclear weapons or a powerful emerging arsenal of hypersonic missiles.

In November, Mr Putin himself suggested Russia could deploy submarine-based hypersonic missiles within close striking distance of Washington.

He has repeatedly said that the prospect of Western military expansion in Ukraine poses an unacceptable risk because it could be used to launch a nuclear strike against Moscow with just a few minutes' warning. Russia, he made clear, could do the same.

"From the beginning of the year, we will have in our arsenal a new sea-based missile, a hypersonic one," Mr Putin said, referring to a weapon that travels at more than five times the speed of sound and could likely evade existing missile defences.

In an apparent reference to the US capital, he added: "The flight time to reach those who give the orders will also be five minutes."

Mr Putin said he would deploy such missiles only in response to Western moves, and Mr Biden told Mr Putin in their last conversation that the US has no plans to place offensive strike systems in Ukraine.

Russian officials hinted again in recent days about new missile deployments, and American officials repeated that they have seen no moves in that direction. But any effort to place weapons close to US cities would create conditions similar to the 1962 crisis that was the closest the world ever came to a nuclear exchange.

Moving missiles, however, is obvious to the world. And that is why, if the conflict escalates further, American officials believe that Mr Putin could be drawn to cyber attacks - easy to deny, superbly tailored for disruption, and amenable to being ramped up or down, depending on the political temperature.

Mr Putin does not need to do much to insert computer code, or malware, into US infrastructure; the Department of Homeland Security has long warned that the Russians have already placed malware inside many US power grids.

The Biden administration has sought to shore up US systems and root out malware. The nation's biggest utilities run an elaborate war game every two years, simulating such an attack. But much of corporate America remains far less protected.

The fear is that if sanctions were imposed on Moscow, Mr Putin's response could be to accelerate the kind of Russia-based ransomware attacks that hit Colonial Pipeline, a major beef producer, and cities and towns across the country last year.

The FSB, Russia's powerful security service, on Friday announced the arrest of hackers tied to the REvil ransomware group - a gang connected to some of the most damaging attacks against US targets, including Colonial Pipeline.

The move was welcomed by the White House, but it was also a signal that Moscow could flip its cyber warriors on or off at will.