Ukraine is reinforcing security measures at its nuclear power plants in response to Russian threats, Ukraine's ambassador told the UN atomic agency in a letter seen by AFP on Wednesday.

"Illegal actions of the Russian armed forces on the Ukrainian territory and the threat of force amount to a grave threat to security of Ukraine with its potential consequences for its nuclear power infrastructure," Ihor Prokopchuk said.

"Under these circumstances, the competent authorities of Ukraine make every effort to ensure physical security, including reinforced physical protection of 15 power units in operation at four sites of Ukrainian (nuclear power plants)," Prokopchuk said in the letter to International Atomic Energy Agency chief Yukiya Amano.

Prokopchuk added however that the "consequences of the use of military force by the Russian Federation" against the former Soviet state "will be unpredictable."

"Numerous efforts of the Government of Ukraine to establish bilateral contacts with the Russian counterparts have not been responded to so far," he said.

He urged Amano to "urgently raise the issue of nuclear security with the authorities of the Russian Federation."