Russian media cited an unnamed source on Sunday as saying that Ukraine was close to building a plutonium-based "dirty bomb" nuclear weapon, although the source cited no evidence.

Russian President Vladimir Putin ordered an invasion of Ukraine on Feb. 24, with the aim to "demilitarise" and "denazify" its pro-Western neighbour and prevent Kyiv from joining NATO.

The West, dismissing that rationale as a pretext, has responded with harsh sanctions on Moscow and heavy military and other aid to Kyiv.

The TASS, RIA and Interfax news agencies quoted "a representative of a competent body" in Russia on Sunday as saying Ukraine was developing nuclear weapons at the destroyed Chernobyl nuclear power plant that was shut down in 2000.