President Joe Biden and other top US officials played down the threat of Russia's "dangerous" nuclear mobilization Monday as the war in Ukraine intensified with more arms being sent to Kyiv by the West.

Asked if Americans should be worried about nuclear war after President Vladimir Putin said he was putting his strategic forces on alert, Biden gave a calm "no" in response.

State Department spokesman Ned Price said Washington sees "no reason" to change the alert levels of the US nuclear force, and a senior defense official said the Pentagon had not seen any palpable shift by the Russians despite Putin's Sunday announcement.

The Pentagon continues to "review and analyze and monitor" Russia's posture, said Defense Department spokesman John Kirby.

Secretary of Defense Lloyd Austin "is comfortable with the strategic deterrence posture of the United States and our ability to defend the homeland," Kirby told reporters.

– 'Provocative rhetoric' –

On Sunday, four days after Moscow launched the invasion of Ukraine, Putin announced that he had ordered his military chiefs "to put the deterrence forces of the Russian army into a special mode of combat service," using a reference to the country's massive nuclear weapons infrastructure.

US officials called the order "dangerous" and "escalatory."

But on Monday officials at the US State and Defense Departments said they were still trying to determine just what action was being taken in response to the order.

"This sort of provocative rhetoric … is dangerous, adds to the risk of miscalculation, (and) should be avoided," Price said.

"We have long agreed — the United States and the Russian Federation — that a nuclear use would have devastating, devastating consequences," said Price.

– Russians slowed, not stopped –

One Monday Biden hosted a video conference with allies and partners supporting Ukraine in its fight against what the White House called "Russia's unjustified and unprovoked war."

In the secure meeting, the group discussed "coordinated efforts to impose severe costs and consequences to hold Russia accountable" while trying to maintain global economic stability, including keeping energy prices down, the White House said.

The US and NATO allies continued to ship in to Ukraine munitions that Price said would help them battle both Russian armored vehicles and air power.

He declined to confirm a report that Washington, following Germany's example, was delivering shoulder-mounted Stinger missiles that can bring down helicopters and slower-moving jets, after denying them to the Ukrainians for months.

In an assessment of the progress of Russian forces, Pentagon spokesman Kirby said fuel shortages, other logistical issues, and unexpectedly stiff resistance from Ukraine's military had significantly slowed the Russian advance.

"It's clear the Russians had not made the progress that they wanted to make by day five," said Kirby.

"They have faced setbacks. And they have faced resistance," Kirby told reporters.

Nevertheless, he said, even if they have been slowed the Russians have an overwhelming force — 25 percent of which remains just outside Ukraine poised to move — that is determined to seize key cities and oust the Ukrainian government.

"Make no mistake, Mr Putin still has at his disposal significant combat power. He hasn't moved all of it into Ukraine," Kirby said.

"They have suffered setbacks, but I don't think we can just assume that they're going to stay set back."

"They continue to want to move on Kyiv, to capture Kyiv," he said.

Russia's nuclear force, the world's biggest
Paris (AFP) Feb 28, 2022 –

President Vladimir Putin's order to place Russia's strategic forces on high alert has served as a stark reminder that he commands what experts agree is the world's biggest nuclear arsenal.

Here is a snapshot of Russia's nuclear capabilities, and how a strike might be launched.

— The most warheads —

Russia owns the greatest number of nuclear warheads of any country, although the United States has more deployed, or immediately usable, warheads.

The SIPRI peace research institute in Stockholm has counted 6,255 Russian warheads against 5,550 for the US. China follows far behind with 350 and France with 290.

These figures, although widely accepted, are nevertheless estimates, notably because not all nuclear-capable weapons systems actually carry nuclear warheads.

According to the Nobel peace prize-winning International Campaign to Abolish Nuclear Weapons (ICAN), Russia spent $8.0 billion in 2020 on the building and maintenance of its nuclear arsenal.

— Chain of command —

The Russian constitution gives the president control over nuclear weapons, but the transmission of any order to use them, and the authentification of that order, also involve the defence minister and the armed forces' chief-of-staff.

How such a scenario would play out exactly "we don't know", said Pavel Podvig, a Russian independent military expert.

The two subordinates have "no right to veto, but there is still some kind of deliberation process," he said, adding: "It's not like there is a button on the president's table, there is a procedure."

Even if a nuclear strike order should come down, there is still the question of whether the armed forces would comply.

"They are not mad and they are not sectarians," said Pavel Luzin, a Russian military expert based in Moscow at Riddle, a think tank.

Many Russian government officials may well agree with western analysts who say that Russia would have much more to lose than to gain if it unleashed nuclear war.

"I personally don't think the Russian military elite will cheer at the idea of limited nuclear use in or about Ukraine," tweeted Kristin Ven Bruusgaard at Stanford University's Center for International Security and Cooperation.

"But who will break the news to Putin that this may in fact not work?", she added.

— Doctrine and reality —

Putin in 2020 adopted a doctrine for the possible use of nuclear weapons, experts Hans Kristensen and Matt Korda recalled in last week's issue of the "Bulletin of the Atomic Scientists".

According to the doctrine, there are four acceptable justifications for going nuclear: A ballistic missile attack against Russia or an ally, the use of a nuclear weapon by an enemy, an attack on a Russian nuclear weapons site, or any attack threatening the existence of the state.

They quoted Putin as saying that Russia — which claims it has modernised close to 90 percent of its nuclear arsenal — could never allow itself to "stand idle" in the face of potential enemies.

"You stop for one second and you start falling behind immediately," they quoted Putin as saying.

They added that Russian thinking may already have gone beyond the published doctrine.

"For example, officials explicitly threatened to use nuclear weapons against ballistic missile defense facilities, and in regional scenarios that do not threaten Russia's survival or involve attacks with weapons of mass destruction," they said.

"The real doctrine," they added, "goes beyond basic deterrence and toward regional war fighting strategies, or even weapons aimed at causing terror."