The UN's International Atomic Energy Agency IAEA will carry out an "independent verification" of Russian allegations concerning the production of so-called dirty bombs at two sites in Ukraine, it said on Thursday.
Moscow has accused Ukraine of preparing to use a dirty bomb — a conventional weapon laced with radiological or chemical substances — against Russian forces.
Kyiv and its Nato allies have denied the claims and said Russia may be using the allegations as cover to conduct its own dirty bomb attack under a "false flag" operation that it could use to justify using tactical nuclear weapons as it faces setbacks in eastern and southern Ukraine.
IAEA head Rafael Grossi said the nuclear watchdog would carry out verifications this week at two sites in Ukraine, after a written request from Kyiv to send inspectors. He said IAEA experts should reach a conclusion within "days".
The IAEA would work "to detect any diversion of nuclear material under safeguards, any undeclared production or processing of nuclear material at the two locations and assure that there are no undeclared nuclear material and activities".
Also at the UN Security Council, Russia once accused Kyiv and the US of operating biological weapons programme in labs in Ukraine.
The UN said it was not aware of any such programme, contrary to Russian accusations.
"This remains the case today," said Adedeji Ebo, deputy to the UN high representative for disarmament affairs.
US ambassador to the UN, Linda Thomas-Greenfield condemned Russia for calling a third meeting this week to discuss Ukraine, saying it was a "colossal waste of time".
"It is Moscow that has a well-documented history of using chemical weapons and shielding the Assad regime, which has repeatedly used chemical weapons, from accountability," Ms Thomas-Greenfield said, referring to Syrian President Bashar Al Assad, who has gassed his own people.
"But here’s the difference: We have ample evidence of our claims," she added.
Russia is now pushing for a formal inquiry and has drafted a Security Council resolution to set up a commission, made up of all 15 council members, to investigate its claims.
"This is our right regardless of whether you like it or not, so let's act within the legal area and leave the political assessment outside of this," said its UN envoy, Vasily Nebenzya.
Ms Thomas-Greenfield said Russia's accusations were "pure fabrications brought forth without a shred of evidence" and that there are no Ukrainian biological weapons laboratories supported by the US administration.
Asked if he was still very concerned about the possibility of a nuclear accident, Mr Grossi stressed the need for a protection zone and expressed his frustration that discussions over the matter are "taking far too long".
"We should have concluded already, we should have established a protection zone and this was also one of my main messages to the distinguished members of the council," he said.