(NewsNation) —  There are credible concerns Russian forces will deploy chemical attacks in Ukraine, U.S. officials have said.

In light of this, Kyiv’s deputy mayor asked the international community for 200,000 more gas masks on top of what other countries have already pledged to give.

Russia has notably used chemical weapons on people before, such as Alexei Navalny, a Russian opposition leader, lawyer and anti-corruption activist who’s spoken out against Russian President Vladimir Putin.

Last month, Russians used tear gas to disperse Ukrainians protesting the occupation of their city.

Now, there are imminent concerns Russia might do this again, particularly in cities such as Mariupol, which has already been devastated by the war.

There are two main types of chemical agents. Those specifically used for battle, like sarin nerve gas, are outlawed by international law. But there are also dual-use chemicals, like tear gas or chlorine. While tear gas is outlawed for war, it can be used for law enforcement purposes, according to USA Today.

As one expert in The Conversation said, controlling the production and distribution of these dual-use chemicals is much harder than dedicated chemical weapons. There can also be difficulties demonstrating the dual-use chemicals were intended to be used as weapons.

The Conversation reports that should a chemical attack happen, a “robust” investigation will be necessary to hold those responsible accountable.