Inspectors of the International Atomic Energy Agency (IAEA) have "begun -- and would soon complete -- verification activities at two locations in Ukraine", the agency said in a statement.

IAEA Director General Rafael Grossi said he would later this week provide "his initial conclusions about the latest verification activities at the two sites", the statement added.

The inspections follow a written request from the Ukraine government to send IAEA teams there.

Russia has accused Ukraine of preparing to use dirty bombs against Moscow's troops, but Kyiv suspects Russia might itself use a dirty bomb in a "false flag" attack, possibly to justify use of conventional nuclear weapons by Moscow as it finds itself on the back foot in eastern and southern Ukraine.

The agency said last week it had inspected "one of the two locations a month ago and no undeclared nuclear activities or materials were found there."

A dirty bomb is a conventional bomb laced with radioactive, biological or chemical materials which are spread in an explosion.

Last Thursday Russian President Vladimir Putin called for the IAEA to inspect Ukraine's nuclear sites "as fast as possible".

The IAEA in Monday's statement also urged the release of an employee of the Zaporizhzhia nuclear power plant, who was detained some two weeks ago.

The plant -- Europe's largest atomic facility -- was captured by Russian troops in March in the early days of the invasion.

Grossi again stressed the plant's "precarious situation", including his concern for "the increasingly difficult and stressful working conditions for the plant's Ukrainian operating personnel".

Another employee detained some two weeks ago was released recently, the statement said.

Ukraine claims Moscow's forces are "kidnapping" the plant's staff and said recently that around 50 employees were held in "captivity".

Russia and Ukraine have accused each other of shelling the plant, sparking fears of a nuclear disaster.

The IAEA has been in contact with both Moscow and Kyiv to set up a security zone around the plant.