The Iranian president made the remarks in a telephone conversation with his Russian counterpart Vladimir Putin.

"The eastward expansion of NATO is a source of tension," President Rayeesi said during the phone talk.

"The continued expansion of NATO is a serious threat against the stability and security of independent countries in various regions” of the world," the Iranian president added.

The Russian president, for his part, said what is happening in Ukraine is "a legitimate response" to several decades of violation of security agreements by the West and its efforts to undermine his country’s security.

The phone call came amid Russia's military operation in the eastern parts of Ukraine, which started after Putin said he had ordered the Russian Federation's military to carry out a “special military operation” in the Donbass region.

Putin’s order came after the leaders of the self-proclaimed republics of Donetsk and Lugansk in the Donbass region asked the Kremlin for military assistance in response to what they called “Ukrainian aggression". 

In 2014, Ukraine’s two regions of Donetsk and Lugansk were turned into self-proclaimed republics by ethnic Russians, leading to a bloody conflict between the government forces and the armed separatists.

On Monday, Putin signed a decree recognizing the breakaway Lugansk and Donetsk regions as independent republics. The recognition followed an address in which he referred to eastern Ukraine as “ancient Russian lands” being “managed by foreign powers.”