The U.S. warned Ukraine of potential chemical assault, Blinken says

As analysts seek to determine whether Russian troops used chemical weapons this week in Mariupol, U.S. Secretary of State Antony Blinken acknowledged that his office had warned Ukraine that chemical-based substances might be employed against them in the increasingly brutal assault.

"We had credible information that Russian forces may use a variety of riot-control agents, including tear gas, mixed with chemical agents, that would cause stronger symptoms to weaken and incapacitate entrenched Ukrainian fighters and civilians as part of the aggressive campaign to take Mariupol," Blinken told reporters on Tuesday. "We shared that information with Ukraine as well as with other partners."

Blinken's remarks came in the aftermath of unconfirmed reports that Ukrainians in Mariupol were attacked with chemicals. 

The reports emerged Monday on social media, as people claimed to have knowledge of a chemical attack. The reports commenced when a Ukrainian battalion stationed in Mariupol claimed that Russian forces used an unmanned aerial vehicle to dispense poison onto the city. The reports described different types of substances, including white phosphorous and sarin — none of which have been documented.

The Pentagon on Monday night said it was aware of the claims, but could not verify them.

"We are aware of social media reports which claim Russian forces deployed a potential chemical munition in Mariupol, Ukraine,"  Pentagon spokesman John Kirby said in a statement. "We cannot confirm at this time and will continue to monitor the situation closely."

Neither Ukraine nor the West have announced definitive proof, but said that the charges are serious.  

"These reports, if true, are deeply concerning and reflective of concerns that we have had about Russia's potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine," Kirby said.

President Joe Biden last month said that the U.S. would respond if Russia launched a chemical attack against Ukraine.

"It would trigger a response in kind," Biden said while speaking at an emergency NATO summit. He did not elaborate on what the response would entail.

Analysts on Tuesday had not yet pinpointed what substance was used in Monday's alleged attack. One report described the substance as being attached to a small weapon that was dropped from a drone and that caused three people to become sick.

Ukrainian President Volodymyr Zelensky described the incident as "preparation for a new stage of terror" against Ukraine. "One of the mouthpieces of the occupiers stated that they could use chemical weapons against the defenders of Mariupol," he said. "We take this as seriously as possible."

The State Department continues to investigate, Blinken said.

"We're in direct conversation with partners to try to determine what actually has happened," he said. "So this is a real concern."

The concern predates Feb. 24, when Russian President Vladimir Putin launched an assault on Ukraine. 

"It's a concern that we had from before the aggression started," Blinken said. "I think I've pointed to the possibility that these kinds of weapons would be used, and it's something that we're very, very focused on."

Putin ordered his forces to invade their western neighbor for what he presumed would be a quick, decisive "special military operation" to "demilitarize" Ukraine. Instead, Russian troops faltered, even while launching devastating artillery and rocket barrages against Ukrainian cities.