Alleged and unconfirmed claims of chemical weapons use by Russia in Ukraine has forced a scramble inside the White House to match President Joe Biden’s promise of an “in kind” response while avoiding further escalation of the conflict.

The White House is urging caution, noting that the use of chemical weapons remains unverified. U.S. officials, speaking on the condition of anonymity to discuss a sensitive subject, said they have been running scenario-planning exercises on the possible use of chemical weapons, having publicly raised the alarm that Russian President Vladimir Putin may take such a step. The officials said that military options in Ukraine aren’t on the table — echoing Biden’s repeated position of not wanting to spark World War III.

The word used by multiple U.S. officials who’ve been involved in contingency planning for such an attack for at least a month is “proportional,” meaning America and its allies intend to respond in a manner befitting the potential war crime.

Instead, some suggested America and its allies could impose further sanctions on Moscow or further bolster Ukraine’s defenses with advanced weaponry. Biden aides have also speculated that the use of chemical weapons may be the final impetus for European nations to stop importing Russian energy, funds for which have fueled Putin’s war machine and filled his country’s coffers.

Before doing any of that, the first step is to confirm a Ukrainian military group’s charge that Russia on Monday deployed a chemical substance in Mariupol. The Azov regiment, a frontline fighting unit that has fought Russia in the Donbas since 2014 and has been tied to neo-Nazi groups and white supremacists, said Russian troops dropped a chemical weapon from a drone and poisoned at least three people, though the group said the affected soldiers are not facing disastrous health effects. If true, that’d be the first known use of chemical weapons in the war since Russia invaded Ukraine on Feb. 24.

U.S. and European officials have yet to substantiate the accusation. Experts say a preliminary assessment could be made using photos or videos, if they exist, while U.S. or Western officials on the ground collect samples for more conclusive verification. Ukraine could also invite the Organization for the Prohibition of Chemical Weapons, a global watchdog headquartered in the Netherlands, to send a rapid-response team to the site for investigation.

Officials cautioned Tuesday that such a determination may not be imminent. It may take some time to assess if chemical weapons were used, just as it did during the conflict in Syria back in 2013.

“There’s no independent verification in that area, so it’s likely to be a long time,” a European official told POLITICO. There are a “host of difficulties” in verifying the claims, a senior U.S. defense official told reporters Tuesday. “These are difficult things to prove even when you are more proximate, and we are not.”

Additionally, U.S. officials raised questions about the credibility of the Azov regiment, noting that the far-right group might be eager to provoke a larger confrontation. They also noted that Ukrainian officials, who have been quick to accuse Russia of atrocities, have not definitively declared that illicit weapons were used.

To that point, during his Monday address Ukrainian President Volodymyr Zelenskyy did not confirm chemical weapons had been deployed, but did say he took the recent threat of their use in Mariupol by Russian-backed separatists “as seriously as possible.” But Anton Gerashchenko, an adviser to the minister of Internal Affairs in Ukraine, hours beforehand tweeted that “Chemical weapons are used” in Mariupol.

Ned Price, the State Department spokesperson, told CNN hours later that the U.S. is working with Ukraine “to try and determine what exactly has transpired here.” British foreign minister Liz Truss added on Twitter that, “Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account.”

The U.S. has long warned that Russia might launch chemical weapons in Ukraine, prompting Biden to tell reporters in Europe last month that his administration would act swiftly if Putin’s troops went that far. “The nature of the response would depend on the nature of the use,” he said, adding “it would trigger a response in kind.”

It’s unclear as of now what the administration deems a “proportional” response to the alleged chemical weapons use in Mariupol. Publicly, Western officials condemn the use of all chemical weapons. But privately they acknowledge that there is a wide-range of lethality in such weapons — in other words, that there’s a big difference between a canister of chlorine and a sarin bomb dropped on a school. More severe consequences, they note, would be doled out in response to the potential use of more dangerous weapons.

What is clear, though, is that some response seems imminent were the international community to verify the Azov regiment’s accusation. “Any confirmed use of prohibited chemical weapons would trigger severe consequences for Russia,” said Andrew Weber, formerly the Pentagon’s top nuclear, biological and chemical weapons official during the Obama years.

The current moment echoes former-President Barack Obama’s “red line” in Syria, in which he pledged that chemical weapons use by Syrian President Bashar al-Assad would trigger a U.S. response.

A year after Obama’s infamous remark, Assad’s forces killed more than 1,400 people with sarin gas. In response, then-Vice President Biden promised a crowd at the American Legion that “those who use chemical weapons against defenseless men, women and children should and must be held accountable.”

Ultimately, the Obama administration struck a deal with Russia to remove 600 metric tons of Syria’s chemical stockpile. Biden praised that decision at the time, crediting the White House for moving the world to act in the face of a “fundamental violation of human rights.” But the merits of the deal were soon thrown into doubt when Syrian forces oversaw additional chemical attacks in 2017 and 2018.

In response to those assaults, then president Donald Trump authorized strikes on Syrian targets.

“We cannot allow atrocities like that,” he said ahead of the second response.

Those strikes were largely symbolic responses. In the first instance, the Trump administration fired missiles at a Syrian air base from which planes had dropped the chemical weapons — but gave advance notice to Russia to keep assets away from the targets. In so doing, it didn’t ignite a larger global conflict, but it also didn’t destroy the entirety of Syria’s chemical weapons program.

Current administration officials insist that the situation is dramatically different now and that Biden has made no such red-line declaration either.

Striking Russia would be far more dangerous than hitting Syrian government targets. Moscow, armed with the world’s largest nuclear arsenal and advanced cyber capabilities, could respond in a way that escalates the conflict outside of Ukraine’s borders. As such, what is being considered now as a response to a confirmed chemical weapons attack are new sanctions, more weapons shipments to Ukraine or even a cyberattack.

“We will select the form and nature of our response based on the nature of the action Russia takes, and we’ll do so in coordination with our allies. And we’ve communicated to the Russians … that there will be a severe price if Russia uses chemical weapons,” national security adviser Jake Sullivan told reporters in March, backing Biden’s comments.