Defense Department press secretary John Kirby said the Pentagon was closely monitoring reports Monday evening of a possible Russian chemical weapons attack on the besieged Ukrainian city of Mariupol.

If Russia were found to have used chemical weapons in the strategic holdout city, it would represent a major escalation of the conflict and present a direct challenge to NATO to act.

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

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”, - said Kirby.