Vehicles used by Russian forces would have raised radioactive dust clouds and soldiers also dug trenches in the most contaminated part of the site, Ukraine's nuclear agency Energoatom said.

The power station, the site of the world's worst nuclear disaster in 1986, was taken back under the control of Ukrainian forces on Thursday as Russia withdrew from areas north of the capital Kyiv.

"All the equipment at the Chernobyl power station is functioning. The control and radiation monitoring systems function as usual," power station chief Valery Seida said in a statement.

But he said the soldiers may have been exposed.

"The thick dust that their vehicles raised into the air and the radioactive particles it contains can easily penetrate into the Russians' organism through their lungs," Seida said.

Since they also dug trenches in the area "it is entirely possible that they have been exposed to considerable levels of radiation," Energoatom said.

Russia's capture of Chernobyl in the early days of its invasion of Ukraine, which began on February 24, raised worldwide fears over nuclear leaks.

Speaking to reporters in Warsaw, Ukrainian Foreign Minister Dmytro Kuleba said Russia had shown "irresponsibility" in its actions around Chernobyl.

"Russia showed irresponsibility on all fronts, from its refusal to allow staff at the power station to fully carry out their duties to digging trenches in the contaminated area," he said.

"The Russian government... will have to answer to the mothers, sisters and wives of these soldiers and explain to them why it forced them to be exposed to these risks," the minister said.

Chernobyl's reactor number 4 at Chernobyl exploded in 1986 and is now covered by a double layer of sarcophagus -- the first built by the Soviet Union and another that was only completed in 2019.

The power station's three other reactors were successively closed down, with the latest shutting off in 2000.