President-elect Donald Trump called for an expansion of America's nuclear capabilities on Thursday, in a tweet that alarmed nonproliferation experts who said a boost to the United States' arsenal could fuel global tensions.

In his Twitter post, Trump said: "The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes."

He gave no further details.

It was not clear what prompted his comment. However, earlier on Thursday, Russian President Vladimir Putin said that Russia needed to "strengthen the military potential of strategic nuclear forces".

On Friday, Putin said Trump's tweet was "nothing special."

Trump spokesman Jason Miller said Trump's comments were not meant to be read as a new policy proposal.

Experts wondered whether Trump's tweet meant he wanted to breach limits imposed on US strategic weapons.

"It is completely irresponsible for the president-elect to make changes to US nuclear policy in 140 characters and without understanding the implications of statements like 'expand the capacity'," said Daryl Kimball, executive director of the Arms Control Association.

"He must have leaders around the world trying to guess what he means," Kimball added.

"This is bush league."