WASHINGTON: The United States cannot confirm the use of chemical agents in Ukraine’s port city of Mariupol at this time, a senior US defense official said on Tuesday.

Ukrainian Deputy Defense Minister Hanna Malyar said the government was checking unverified information that Russia may have used chemical weapons while besieging Mariupol.

“We cannot confirm the use of chemical agents at this time,” the US official, speaking on the condition of anonymity, told reporters.

The official added the United States had no information to support the movement of chemical agents by Russia in or near Ukraine.

Ukraine repulses attacks in east as Russia presses to control Mariupol

US President Joe Biden last month said Russia’s unsubstantiated accusations that Kyiv had biological and chemical weapons suggested Russian President Vladimir Putin was considering using them in Ukraine.

Chemical weapons production, use and stockpiling is banned under the 1997 Chemical Weapons Convention. Although condemned by human rights groups, white phosphorous is not banned under the convention.

Russia’s defence ministry did not immediately respond to a Reuters request for comment. Russian-backed separatist forces in the east denied using chemical weapons in Mariupol, the Interfax news agency reported.

Ukraine rejects Russian ultimatum to surrender Mariupol

After their troops got bogged down in the face of Ukrainian resistance, the Russians abandoned their bid to capture the capital Kyiv and are redoubling their efforts in the east where the US official said a Russian convoy was now about 60 kilometers (37 miles) from the eastern town of Izyum.