Vladimir Putin and Donald Trump have both hinted at possible expansions of their countries' nuclear weaponry.
Video report by ITV News Washington correspondent Robert Moore:
In a speech on Thursday discussing military activities through the year, the Russian president called for further improvements to ensure the country could "neutralise any military threat".
Later on, the US President-elect - taking to his favoured form of communication, Twitter - did not address Mr Putin's comments directly, but said America should "greatly strengthen and expand its nuclear capability" until "the world comes to its senses regarding nukes."
The remarks immediately sparked a flood of responses on the social networking site voicing fear over what the outcome of such a program might be.
Others mocked him for talking of expansion despite serving President Barack Obama's $1 trillion investment in the nuclear arsenal.
But later, Mr Trump's spokesman Jason Miller appeared to backtrack on what was said - insisting that the President-elect had, in fact, been referring to the need to prevent nuclear weapons from falling into the wrong hands.
Speaking earlier in the day, Mr Putin his army's preparedness has "considerably increased" in the past 12 months, and said they had successfully demonstrated its strength in Syria - and said the country needed to remain alert to political changes.
"We need to strengthen the military potential of strategic nuclear forces, especially with missile complexes that can reliably penetrate any existing and prospective missile defence systems," he said.
"We must carefully monitor any changes in the balance of power and in the political-military situation in the world, especially along Russian borders, and quickly adapt plans for neutralising threats to our country."
Russia's actions in Syria had also awakened interested from foreign countries in buying Russian-built weapons, he added.
According to the non-partisan Arms Control Association, the US currently has an estimated 7,100 nuclear warheads and Russia has 7,300 - meaning that between them, the two countries own some 90 per cent of the world's nuclear inventory.
By comparison, the UK has 215 nuclear weapons, France has 300 and North Korea has eight.
During his campaign, Mr Trump told a Fox News interviewer that Japan and South Korea should consider arming themselves with nuclear weapons to help the US fight North Korea.
He later retracted that statement.