MOSCOW - With Russian troops massed along the border with Ukraine, President Vladimir Putin is expected to issue President Joe Biden an ultimatum during their video meeting Tuesday: Guarantee that NATO will never expand into Ukraine, or Russia might soon launch an offensive against its neighbor.

The Kremlin has said it wants written guarantees from the United States and its allies in NATO that the military alliance will not expand east - both in terms of membership and Western forces.

The video call comes during an unprecedented low point in U.S.-Russia relations, especially over Ukraine. The White House has threatened Russia with "serious consequences" - believed to be sanctions that would cut the country off from the global financial system - if it pursues military action against Ukraine.

Putin plans to outline Russia's proposals to Biden, Kremlin spokesman Dmitry Peskov said Monday, and their talks are expected "to be quite long and substantive."

After a meeting with Secretary of State Antony Blinken in Stockholm last week, Russian Foreign Minister Sergei Lavrov said he did not even want to speculate "about whether the West would refuse to consider [our proposals]. To my mind, everyone has heard President Putin and grown aware that [they] are serious."

Biden, however, said Friday that he "won't accept anybody's red line."

The U.S. president is slated to speak with key European allies on Monday ahead of the call to ensure strong allied unity and transatlantic solidarity going into the talks, a senior Biden administration official said Monday during a briefing with reporters. Blinken will speak to Ukrainian President Volodymyr Zelensky ahead of the call, and Biden will speak with the Ukrainian leader in the days after, the official said.

During the call, Biden plans to make clear to Putin what costs the United States and its allies will impose on Russia if an invasion occurs but also will offer a diplomatic pathway that could address Russian concerns about NATO activities, as well as U.S. and European concerns about Russian activities, the official said, speaking on the condition of anonymity to discuss sensitive diplomatic matters.

Still, the official suggested that a written guarantee from Washington that NATO will not expand to include Ukraine was a nonstarter, noting that the United States believes that every sovereign nation should have the right to make its own decisions about its security.

"We don't think talk of red lines is helpful, and as the president has said, we are not going to operate according to that logic of accepting anyone's red lines," the official said.

Washington has agreed with its European allies on measures that would cause "significant and severe economic harm" to Russia if Putin chooses to proceed, the official said, declining to go into details.

An invasion would prompt the United States to step up its support for Ukraine and send "additional forces and capabilities" to NATO allies on Europe's eastern flank, the official said. Washington is working through a "prudent planning" of what the United States would have to do in the event of such an escalation to ensure the security of those NATO allies, the official added.

The official strongly suggested that the White House was not considering sending U.S. combat forces into Ukraine should Russia mount another invasion.

"The United States is not seeking to end up in a circumstance in which the focus of our countermeasures is the direct use of American military force, as opposed to a combination of support for the Ukrainian military, strong economic countermeasures and the substantial increase in support and capability to our NATO allies to ensure that they will remain safe," the official said.

Putin "is not bluffing, and he is prepared to undertake a military operation against Ukraine," said Tatiana Stanovaya, the head of the R.Politik think tank.

But that Putin will meet with Biden amid heightened tensions with the West shows that Putin considers Biden someone "who is ready to talk seriously about Russian concerns," Stanovaya said.

The two last met in June in Geneva, and though that summit resulted in few breakthroughs, it has led to more communication between the White House and the Kremlin, including several top administration officials visiting Moscow in recent months.

The Geneva meeting also came amid an earlier Russian troop buildup around Ukraine, though the forces were later partially pulled back. That spring show of force was dismissed by most analysts as testing the new Washington administration's support for Ukraine.

The latest troop movements have caused greater alarm in the West. U.S. intelligence has concluded that the Kremlin could be planning a multifront offensive as soon as early next year involving up to 175,000 troops, according to U.S. officials and an intelligence document obtained by The Washington Post.

The United States doesn't know whether Putin has issued an order or made a decision about a Ukraine offensive, the senior administration official said, but U.S. intelligence is tracking a buildup that is consistent with planning for such an operation.

Russian disinformation activity in cyberspace regarding Ukraine also has increased in the way it did in the run-up to Russia's 2014 invasion, the official said.

The rhetoric from Russia is that it's the one being threatened. In an address to Russia's Foreign Ministry Board in November, Putin warned that the West has a "superficial approach to our warnings about red lines." He noted that the United States and its allies supply Kyiv with lethal weapons, conduct "provocative" military exercises in the Black Sea and fly strategic bombers just 12 miles from Russia's borders.

He has also warned against stationing missile defense systems in Ukraine similar to those in Romania and Poland, claiming that they could be secret offensive weapons capable of reaching Moscow within 10 minutes.

"For him, it's all about personal survival," said Pavel Felgenhauer, a Moscow-based military analyst. "If you're sitting in the Kremlin for more than 20 years ruling Russia, you get paranoid."

Putin could insist on NATO stopping all military cooperation with Ukraine, including training exercises, Stanovaya said. The United States also has provided military support to Ukraine, including equipment such as patrol boats and Javelin antitank missiles. Biden administration officials have also said they are looking for ways to strengthen military deterrence on NATO's eastern flank.Putin "would like for Ukraine to know firmly that it will never be part of NATO and the West must be clear about it," Stanovaya said. "And it means that Western politicians should not give any hopes or inspire Ukrainian elites about the prospects for NATO."

Practically, Kyiv's aspirations to join the military alliance have long appeared unlikely to be fulfilled. Despite Zelensky's renewed push this year to have Ukraine's ascension greenlighted, the country hasn't received a Membership Action Plan, the first step. Biden in June said that "school's out" on Ukraine's candidacy because the country still has to "clean up corruption" before it is considered.

That is no secret to Moscow. Dmitry Kiselyov - the host of the state television program "Vesti Nedeli" ("News of the Week") and a leading figure in Russia's propaganda hierarchy - said Sunday that "Ukraine is taking the bait of the irresponsible nudging from the West, believing that someone there is ready to die for it. They are not."

Some analysts have speculated that the Kremlin's insistence on guarantees from NATO - which are unlikely to be given, and even if they are, unlikely to be trusted - could just be the pretext for plans Russia has already set in motion.

In a commentary for the Carnegie Moscow Center, Alexander Baunov said that "these overwrought statements are an attempt to absolve itself of responsibility for its future actions: alas, Moscow had warned of the coming storm, and called for action, but to no avail."

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The Washington Post's Mary Ilyushina and Natalia Abbakumova contributed to this report.