Russian President Vladimir Putin on Sunday ordered his defence chiefs to put the country’s nuclear arms unit on high alert, Reuters reported.

He cited the “aggressive” statement by The North Atlantic Treaty Organization, or NATO, and economic sanctions levied against Moscow as the reason behind his move.

Putin’s order came during a meeting with his Defence Minister Sergei Shoigu and Chief of the General Staff of the Armed Forces Valery Gerasimov, The Guardian reported.

“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 during a televised address, according to Agence France-Presse.

The measure came as Russia continued its invasion of Ukraine. The Russian offence has entered its fourth day with its forces entering Kharkiv, Ukraine’s second-largest city.

Several countries such as the United States, the United Kingdom, European Union nations and other countries have imposed sanctions against Russia, including cutting off some Russian banks from the Society for Worldwide Interbank Financial Telecommunication, or Swift, a global interbank payments system.

During Sunday’s meeting, Putin said that western nations were putting “illegitimate sanctions” on Russia.

“Senior officials of leading NATO countries also allow aggressive statements against our country,” Putin told the officials during the meeting, according to Agence France-Presse.

Last week, Putin had warned foreign nations to not interfere in Russia’s invasion of Ukraine. “Those who attempt to interfere with the Russian action will witness consequences they have never seen” he had said.