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

International tensions are already soaring over Russia's invasion of Ukraine, and Putin's order will cause further alarm.

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."

Fighting continues as Russian troops enter Ukraine's 2nd largest city of Kharkiv

Defence Minister Shoigu replied: "Affirmative."

The Russian president on Thursday ordered the invasion of Ukraine, sending shockwaves around the world.

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.