World Bulletin/News Desk

Ukraine is reinforcing the protection of its nuclear power plants, it told the U.N. atomic watchdog on Tuesday, because of "a grave threat to the security" of the country posed by the Russian military.

Ukraine has 15 nuclear power reactors in operation, accounting for nearly 44 percent of its electricity production in 2013, according to the International Atomic Energy Agency's (IAEA's) website.

Ukraine's envoy to the IAEA said in a letter to IAEA Director General Yukiya Amano: "Illegal actions of the Russian armed forces on Ukrainian territory and the threat of use of force amount to a grave threat to security of Ukraine with its potential consequences for its nuclear power infrastructure."

Ambassador Ihor Prokopchuk's letter, dated March 4, was circulated among delegations attending a week-long meeting of the IAEA's 35-nation governing board in Vienna. It was given to Reuters by a diplomat from another country.

President Vladimir Putin delivered a robust defence of Russia's military intervention in Crimea on Tuesday and reserved the right to use all options to protect compatriots after the overthrow of Ukraine's Russian-backed President Victor Yanukovich, but he said the use of force would be a last resort.

Prokopchuk's letter to Amano, apparently written before Putin's comments, 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 NPPs (nuclear power plants).

"However, consequences of the use of military force by the Russian federation against Ukraine will be unpredictable."

On Sunday, Ukraine's parliament called for international monitors to help protect its nuclear power plants, as tension mounted with its neighbour.

Prokopchuk urged Amano to "join international efforts in de-escalating the crisis around Ukraine and to urgently raise the issue of nuclear security" with Russia.

Amano said on Monday there were 31 nuclear-related facilities in Ukraine that were being monitored by the IAEA to make sure there was no diversion of material for military purposes, as it does in other countries with nuclear plants.

He said it was the responsibility of individual countries to ensure the security of nuclear power plants.