| A spokesman later said Mr Trump was referring to the need to combat the threat of nuclear weapons |
Donald Trump has called for the US to "greatly strengthen and expand" its nuclear capabilities.
| Russia is stronger than any potential foe, Mr Putin told his advisers |
The US has 7,100 nuclear weapons and Russia has 7,300, according to the US nonpartisan Arms Control Association. Mr Trump's comments came in the form of a tweet, giving no other details.
Hours later, Jason Miller, the communications manager for the Trump transition team, explained he "was referring to the threat of nuclear proliferation and the critical need to prevent it - particularly to and among terrorist organizations and unstable and rogue regimes".
Mr Miller also added that the president-elect "emphasised the need to improve and modernize our deterrent capability as a vital way to pursue peace through strength".
Mr Trump's tweet came after President Putin met with his military advisers to review Russian military activities in 2016.
"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," Mr Putin said.
Russia, he added "must carefully monitor any changes in the balance of power and in the political-military situation in the world, especially along Russian borders", possibly referring to US missile defences in eastern Europe which the Pentagon says are to counter Iran.
During Mr Trump's campaign he referred to nuclear proliferation as the "single biggest problem" facing the world, but also said he could not rule out using nuclear weapons against Europe.
Mr Trump's defeated Democratic rival Hillary Clinton repeatedly cast her opponent during the campaign as too erratic and lacking in the diplomatic skills required to avoid a nuclear war.
She mocked him by saying "a man who can be provoked by a tweet should not have his fingers anywhere near the nuclear codes".