The US does not have confirmation of a purported chemical attack on Ukrainian fighters in the port city of Mariupol, Secretary of State Antony Blinken said Tuesday. 

The top diplomat said neither Washington nor Kyiv are "in a position to confirm anything" about the alleged attack on the besieged city.

But Blinken said the US 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."

"We share that information with Ukraine as well as with other partners, and we're in direct conversation with partners to try to determine what actually has happened," he told reporters at the State Department.

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

An unconfirmed report from a Ukrainian battalion stationed in Mariupol claimed that Russian forces, through the use of an unmanned aerial vehicle, deployed a poisonous substance above the city. There have been independent verifications of the claim.

Russian forces currently encircle the city.

At least 1,892 civilians have been killed and 2,558 injured in Ukraine since Russia declared war Feb. 24, according to UN estimates, with the true figure feared to be much higher.

More than 4.6 million Ukrainians have fled to other countries, with millions more internally displaced, according to the UN refugee agency./aa