Putin orders nuclear forces on high alert

Russian President Vladimir Putin ordered his defence chiefs to put the country's nuclear "deterrence forces" on high alert on Sunday and accused the West of taking "unfriendly" steps against his country.

Moscow has the world's second-largest arsenal of nuclear weapons and a huge cache of ballistic missiles which form the backbone of the country's deterrence forces.

"I order the defence minister and the chief of the general staff of the Russian armed forces to put the deterrence forces of the Russian army into a special mode of combat service," Putin said.

"You see that Western countries are not only unfriendly to our country in the economic sphere - I mean illegitimate sanctions," he added, in a televised address.

"Senior officials of leading Nato countries also allow aggressive statements against our country."

Defence Minister Shoigu replied: "Affirmative."

Ukrainian authorities warned Friday that radiation levels had increased in the Chernobyl exclusion zone since it was seized by invading Russian troops, but the UN's nuclear watchdog said it currently "posed no danger".

Russian ground forces have pressed into Ukraine from the north, east and south but have encountered fierce resistance from Ukrainian troops, the intensity of which has likely surprised Moscow, according to Western sources.

Ukrainian authorities describe some Russian troops as demoralised and exhausted, claiming that dozens have surrendered.