If Russia used nuclear weapons in the Ukrainian war, what would President Joe Biden do?

A half dozen current and former government officials briefed on the issue and several outside experts told NBC News there was no consensus on how the U.S. would respond to a norm-breaking act of destruction that could obliterate a Ukrainian city.

The Biden administration has dealt with this before. There were disagreements about how to react when the Obama administration played a war game with Russia.

Several versions of Russian military doctrine published since 2000 have envisioned the first use of nuclear weapons in response to a conventional threat in a regional war. Military experts say that Russia's smallest warheads are more powerful than the bombs dropped on Nagasaki and Hiroshima.

As the U.S. continues to send more sophisticated weapons to help destroy invading Russian forces, American officials tell NBC News the Biden administration has been thinking about what Putin could do.

William Burns, the director of the CIA, said last month that there was no evidence of Russian plans to use nuclear weapons. We can't take lightly the kind of saber-rattling that the Russian leadership has.

A number of current and former officials were briefed by NBC News. The officials wouldn't speak on the record due to the sensitivity of the planning.

One U.S. official briefed on U.S. government deliberations said that the American response would depend on how the Russians used a nuclear weapon.

There is a demonstration over the Black Sea. There was a strike on Ukrainian troops. A nuclear attack on a NATO country or a devastating blow to a major Ukrainian city are possibilities.

The menu of American options is stark, officials and outside experts say: Stay the course, up the sanctions and keep aiding the Ukrainians, while building an international coalition against Russia that completely isolates the country, launch a conventional military attack on Russian forces inUkraine or Russia, or respond with a The U.S. wouldn't have to respond if a NATO country was hit.

According to some military and intelligence officials, it is unlikely the U.S. will retaliate after a single Russian use of a tactical nuclear weapon. The option of attacking Russian troops in Ukraine or the Russian military unit that launched the nuclear weapon could have serious repercussions, since Russian military leaders might be killed.

I expect the U.S. to enter a war against Russia to stop it from using a nuclear weapon. The order will be signed by Putin.

Two U.S. officials who were briefed on the issue did not agree with one saying that unless they use them on NATO we are probably not going to respond with military force.

Biden wouldn't want to risk a full-scale nuclear war that would destroy American cities. If Putin were to go nuclear, experts believe most other countries in the world would quickly turn against Russia and ostracize it.

An expert at the Quincy Institute for Responsible Statecraft said that the world would stop.

A remote possibility that can’t be taken lightly

Russian tactical nuclear weapons have not been moved out of storage facilities according to national security officials. According to intelligence officials, Putin would consider nukes only if he believed his regime was in danger.

Some in Putin's inner circle have encouraged him to test a nuclear weapon as a show of strength during times when his conventional forces have struggled. If Putin thinks Russia has been backed into a corner, he could choose this option.

After the invasion of Ukraine, Putin put Moscow's nuclear forces on high alert, warning that a direct attack on the country would cause terrible consequences. The U.S. officials told NBC News that they didn't see any changes at the time. Russia's foreign minister warned the West not to underestimate the risk of a nuclear conflict over Ukraine. Putin supporters on Russian state TV talk about a nuclear war with the US and Europe.

There is a backdrop of a Russian nuclear doctrine that has evolved in disturbing ways. Russia stated in a 1993 document that it would use nuclear weapons only when the country was in danger. According to scholars who have looked at it, Russia has the right to use nuclear weapons in response to any weapons of mass destruction used to attack Russia and its allies.

Nuclear weapons can be used in response to large scale aggression using conventional weapons in situations critical to the national security of the Russian Federation. According to experts, the strategy of escalate to de-escalate means that Russia is willing to use nuclear weapons to win a war.

Even though the U.S. nuclear doctrine is similar, experts don't think an American president will ever use nuclear weapons in a regional war. The main purpose of the U.S. nuclear arsenal is to deter a nuclear attack, according to officials. Nuclear weapons can still be used in response to biological or chemical attacks. There are still around 100 nuclear weapons in NATO countries.

Nuclear warning shot?

Officials don't know what could prompt Putin to use a nuke. Is it possible to cement gains on the battlefield? Is it possible to reverse losses. Do you think it's a good idea to stave off a big defeat?

Where is the red line? Is it enough if Ukrainian forces entered Russian territory? Chris Chivvis was the U.S. national intelligence officer for Europe from April 18 to April 21.

In the last 15 years, he said, we have been surprised by Russia several times.

Jeffrey Lewis, a nuclear expert at the Middlebury Institute, said that Russia doesn't have that small of a nuclear weapon. Its nuclear weapons have huge power. The differences between tactical nuclear weapons and strategic ones are in the targets and the goals. Tactical nukes are used to gain advantage on the battlefield while strategic weapons are aimed at military infrastructure.

If Russia decided to use one, its options could include an attack on an air base or other military target, an attack on a Ukrainian city or a test of a nuclear weapon. The weapon could be used as a bomb or a missile.

The nuclear test could be the most attractive option for Moscow.

Only North Korea has tested a nuclear weapon in this century.

Depending on the weather and the location of the test, radioactive material could drift into populated areas in Russia or NATO countries. The last nuclear test of the Soviet Union took place in 1989.

If Russia faced defeat in Ukraine, they could try to force terror on the other side and get the Ukrainians to fold, according to Rose Gottemoeller.

He said that they would be trying to strike terror into the hearts of the Ukrainians.

It was Putin's belief that it would ensure his survival and that he would surrender to the Ukrainians.

Chivvis said that instead of a nuclear exchange with the U.S., Russia could use smaller steps up the nuclear escalatory ladder.

There are scenarios in which Russia could use nuclear weapons that people don't know about. He said that they are not the most likely scenarios to be responsible.

Strategic ambiguity

The Biden administration has avoided spelling out how it would respond if Russia launched a nuclear attack in Ukraine, leaving open the possibility of retaliating with nuclear weapons, conventional forces, or other means.

Alexander Vershbow was deputy secretary general of NATO from 2012 to 2016 and ambassador to Russia from 2001 to 2005.

Biden would have to consider a major military response in support of the Ukrainians.

The dilemma for the US and its NATO allies would be posed by a Russian attack on Ukraine. Western political leaders would have to consider a response that would avoid a full-blown nuclear conflict with Russia but still impose a heavy cost on Moscow.

There are four possible responses, only two of which are plausible, according to an expert on the Russian military.

Staying the course is the real choice, according to him.

The moral high ground could be taken by Biden if he decided that what they were doing was working.

Russia would be more isolated and the international sanctions pressure would increase. The calls for a military response would bedeafening in Washington.

In his book "The Bomb," author Fred Kaplan wrote about a simulation of a Russian nuclear attack on a NATO country during the Obama administration's time in office. Lower level officials decided not to use a nuclear weapon. When the same scenario was presented to Cabinet level officials, they decided that the U.S. had to retaliate with a nuclear attack on Russia.

Cirincione thought that was crazy. A limited nuclear exchange is believed to be possible. You don't want to be in that box because you can't control it.

The track record of the Biden administration suggests it would move cautiously, in consultation with its European allies, and try to avoid plunging the world into a nuclear conflagration, according to former officials.

The administration has faced criticism that it has moved too slowly to send advanced weapons to Ukraine but the White House's supporters say the administration has focused on avoiding actions that could escalate the crisis into a direct clash between Russia and the US.

It's realistic that the U.S. would look for ways to respond to the situation in Ukraine.

She said that the U.S. needs to avoid any kind of nuclear escalation in order to survive.