"Our technical and scientific evaluation of the results we have so far did not show any sign of undeclared nuclear activities and materials at these three locations," said Rafael Grossi, head of the International Atomic Energy Agency (IAEA).

"We will report on the results of the environmental sampling as soon as possible," Grossi added, referring to samples collected by IAEA inspectors.

The inspections at the Institute for Nuclear Research in Kyiv, the Eastern Mining and Processing Plant in Zhovti Kody and the Production Association Pivdennyi Machine-Building Plant in Dnipro followed a written request from Ukraine.

"Over the past few days, the inspectors were able to carry out all activities that the IAEA had planned to conduct and were given unfettered access to the locations," the UN agency's statement said.

Russia accused Ukraine of preparing to use dirty bombs against Russian troops while Kyiv suspects Moscow might itself use a dirty bomb in a "false flag" attack, possibly to justify use of conventional nuclear weapons as ground troops run into setbacks.

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

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